Bar-Bet: Does DOF change with format? [Archive] - Large Format Photography Forum

rdenney

8-Apr-2010, 19:28

Depth of field is identical at a given focal length and aperture.

And given the same subject (focus) distance.

A 300mm lens at f/5.6 on an 8x10, making a full-face portrait at a distance of 2 feet, will be at 1:2 near macro, making an image half the size of the actual face.

A 300mm lens at f/5.6 on a 35mm camera at 2 feet will have exactly the same depth of field, except that the frame will only show the subject's nose (and you'll probably need an extension tube).

Problem is that most people match the contents of the frame, rather than the magnification, and it's the magnification that counts. The backing up that would be required to make a full-face portrait with a 300mm lens on a small camera compared to the 8x10 would have a big effect on depth of field.

There is also the matter than the standard of sharpness is different between the two formats, because the small-format image is expected to require much more enlargement. DOFMaster, for example, assumes an 8x10 print. When assuming larger prints, the circle of confusion used by DOFMaster needs to be scaled inverse to the increase in enlargement.

Rick "noting that large-format has shallower depth of field because it uses longer lenses and (often) closer subject distances" Denney

BetterSense

8-Apr-2010, 20:06

Ivan J. Eberle

8-Apr-2010, 20:33

Along with this discussion of DOF, can I mention how I personally loathe the term "equivalent focal lengths"? What began many years ago as a textbook method to try to get the beginner's head around equipment buying choices for the multitude of formats, becomes handicapping and a cluttered way to think when moving between mulitiple formats in practical use.

So, will it bother anyone if I resist the notion that a 300mm is anything but a 300mm whether on in 135 format, 645 or 4x5 or 5x7 or 8x10?

I arrived at this by first having developed a working familiarity with all manner of focal lengths from ultrawide to supertele in 135mm format for thirty years, before ever venturing into MF and LF. Resultingly I find it preferable to think of a step up in formats as expanding the field of view at any given focal length.

The reason this makes the most sense to me is because relationships between objects at various distances (e.g. background separation) is best determined by focal length, not equivalent field of view/"equivalent focal length". Telephoto stacking effect occurs the same at the same focal lengths (but not at "equivalent focal length").

Likewise, the pleasingly undistorted (less fishy) view of a 35mm focal length lens looks great on 645 format, and doesn't to me have the look of a 21mm, it's supposed equivalent in 135 format. It's also why I'm not so keen on making my 4x5 cameras do-it-all by getting wild with trying to do what other formats achieve much more readily. To get the FOV of the 21mm in 135 or the 35mm in 645 I'd have to go to a 65mm to get something similar in 4x5, but certainly that's not an equivalent DOF by any stretch.

This is especially true when photographing subjects that are not static. Certain things are the forte of 35mm or MF that really aren't done better in LF-- like get great depth of field and stop the apparent motion of waves on the ocean, for instance. Not so attainable at a 65mm focal length, given the constraints of slow films. At any given equivalent field of view, the greater DOF advantage goes to the smaller format.

rdenney

8-Apr-2010, 20:55

1. Relative system aperture (f/stops or numerical aperture)
2. Magnification (more magnification--less DOF)
3. Allowable circle of confusion (CoC)

Yup, but magnification is a function of subject distance and focal length.

Rick "because the bar bet was about focal length, and bar bets are important" Denney

Brian Ellis

8-Apr-2010, 21:19

I'm a scientist and I'm familiar with DOF across many different disciplines only one of which is photography. Photographers have an often flawed and usually overcomplicated understanding of DOF. The very idea that DOF would change with format betrays an incomplete understanding, as if light rays could tell what size your camera is.

Here it is. DOF depends on exactly 3 things. No more. No less.

1. Relative system aperture (f/stops or numerical aperture)
2. Magnification (more magnification--less DOF)
3. Allowable circle of confusion (CoC)

That's it. Nothing else matters. The size of the camera wrapped around the light rays is irrelevant.

If you take any two cameras of any two formats they will have exactly the same DOF at a given magnification and f/stop.

Often people find that they have less DOF with larger formats because they tend to work at higher magnifications to fill up all that film, and the smaller apertures and larger CoC allowed by the large format STILL don't make up for the difference.

You're right that there are only three things that affect depth of field but you didn't get the right three. It's aperture, focal length, and camera-to-subject distance. The size of the circles of confusion depend on those three factors, not the other way around. Depth of field doubles as the f stop number doubles, doubling the camera-to- subject distance quadruples depth of field, and reducing the focal length by half quadruples depth of field, in each case assuming that only one of the three variables is changed (or so said Ansel Adams, who I tend to believe with things like this).

I'm not sure what you mean when you say people "find less DOF with larger formats" because they tend to work at greater magnification. The reason people have less depth of field with larger format cameras is that they have to use longer focal length lenses to make the same photograph as someone with a smaller format camera. E.g., someone using an 8x10 camera has to use a focal length lens that's twice as long as a person using a 4x5 camera to make the same photograph. Longer focal length = less DofF, all other things being equal.

Jack Dahlgren

8-Apr-2010, 22:12

You're right that there are only three things that affect depth of field but you didn't get the right three. It's aperture, focal length, and camera-to-subject distance. The size of the circles of confusion depend on those three factors, not the other way around. Depth of field doubles as the f stop number doubles, doubling the camera-to- subject distance quadruples depth of field, and reducing the focal length by half quadruples depth of field, in each case assuming that only one of the three variables is changed (or so said Ansel Adams, who I tend to believe with things like this).

I'm not sure what you mean when you say people "find less DOF with larger formats" because they tend to work at greater magnification. The reason people have less depth of field with larger format cameras is that they have to use longer focal length lenses to make the same photograph as someone with a smaller format camera. E.g., someone using an 8x10 camera has to use a focal length lens that's twice as long as a person using a 4x5 camera to make the same photograph. Longer focal length = less DofF, all other things being equal.

Am I wrong that discussion of whether it is magnification or focal length and subject distance is kind of like arguing that fuel efficiency is about miles and gallons rather than miles per gallon. Or that aperture should be replaced by iris diameter and focal length. Isn't the math all the same, just different terms?

M = f/f-s, right?

But of course, if there is no argument, it is not a good bar bet.

Jeff Conrad

9-Apr-2010, 01:32

As with so many things, it depends ... here, it depends on what is held constant for the comparisons. Under the "normal" assumption of the "same picture" in both formats (i.e., same camera-to-subject distance, angle of view, f-number, final-image size, and final-image circle of confusion), the DoF is inversely proportional to the format size. Of course, the focal length must be adjusted to give the same angle of view. But apparently, that wasn't the bet. For the conditions stated, the answer depends on the relationship between the captured images and the final images. If each captured image is enlarged uncropped to the same-size final image, the smaller format will have less DoF because of the greater enlargement. But the angles of view will be quite different, so at least to me, the comparison doesn't make much sense. And there's the small detail of the different aspect ratios of the two formats.

The comparison using the same focal lengths in both formats seems to arise most often when comparing full-frame and "cropped" 35 mm formats, probably because many of the same lenses can be used in both formats; the answer again depends on what is assumed in the comparisons. Probably half of what's written on this is nonsense, and the time and energy spent discussing it boggles the mind.

The Wikipedia article on Depth of field (http://en.wikipedia.org/wiki/Depth_of_field#DOF_vs._format_size_2) covers several cases; it's essentially an expansion on what Stroebel did long ago.

It should be borne in mind that the comparisons are approximations that are valid only at medium camera-to-subject distances; the comparisons break down both at large distances and in the macro range.

Sascha Welter

9-Apr-2010, 03:16

Quote:
Here it is. DOF depends on exactly 3 things. No more. No less.
1. Relative system aperture (f/stops or numerical aperture)
2. Magnification (more magnification--less DOF)
3. Allowable circle of confusion (CoC)
(end quote)

Indeed. May I add that point 2 "Magnification" is the total magnification from the object to the size that it will appear in the final art (e.g. print) to the eye?

I learned this in practical terms, many years ago. The task set by a client was to photograph an object that is similar to an open box that you look through. It was to be sharp from front to back, all parts. It was also quite large and more importantly, supposed to fill the frame. The "open box" shape prevents tilting or swinging to be of any help.

I tried with the normal lens, couldn't get enough DOF. Wise guy that I was, "wide angle lenses have more DOF" I had heard, so I take out the 90mm. Now I have to move closer to fill the object (which alters the perspective, not what I wanted) or crop more. Cropping more results in more magnification (assuming you look at a slide on a light table), so the same DOF really.

Next step, I heard that larger format really has less DOF. So I bring out the 6x9. Sure, looks fine on the ground glass... but when you enlarge to a print, it's the same thing again. Point 3, the "allowable circle of confusion" is really the answer to the question of "how large do you want to print this and at what distance will you look at it".

Emmanuel BIGLER

9-Apr-2010, 04:21

Hello all !

To continue Jeff's remarks, being a bit provocative, I would say that DoF can be made totally independant from the format under certain operating conditions.
See this discussion for details
http://www.largeformatphotography.info/forum/showthread.php?t=43468&highlight=dof+independant

My feeling is that the "apparent" increase of DoF in small-format photoraphy is mostly due to the fact that nobody enlarges small-format negatives to the same degree as can be obtained with a large format negative. Or that we never make a side-by-side comparison.
Digital images make the comparisons beteween formats of between film and silicon a bit tricky due to the combination of proprietary, secret pre-processing algorithms in the style of edge-enhancement, combined to all post-processing enhancements applied by the photographer.
All those pre- and post-processing techniques can change dramatically the apparent sharpness of the image on a final print, hence the apparent DoF, and required choice of the Circle of confusion, the key and most controversial parameter in Dof derivations.
Too bad the DoF is not simply a function of focal length, f-number and diagonal of the image !!

bsimison

9-Apr-2010, 04:52

Paul Kierstead

9-Apr-2010, 05:20

This thread reminds me of my professor wife having debates with her academic colleagues. They will essentially all say the same thing, each using slightly different nomenclature, and will argue about it ad infinitum.

Actually, even worse, most will make some set of assumptions that are unstated. They will also introduce misleading statements like "only depends on ..." and then state things which have further dependencies, or are combinations of things; kind of like saying "g only depends on one thing: a+b^(c+d/(e*f))". In every argument, at least one or two doing these things will claim to be a scientist, yet they are making science errors 101 (not stating assumptions, and expanding their factors; not even looking at them, in fact), so one can only assume it is a soft science.

BetterSense

9-Apr-2010, 06:24

You're right that there are only three things that affect depth of field but you didn't get the right three. It's aperture, focal length, and camera-to-subject distance

Focal length and camera-to-subject distance completely describe magnification. Why would I list two quantities when I could list one equivalent quantity?

Dan Fromm

9-Apr-2010, 06:48

Bettersense, film-to-subject distance and focal length define two magnifications for all distances greater than 4f + internodal distance.

When you write "camera-to-subject distance" define what you mean, the term has more than one interpretation.

rdenney

9-Apr-2010, 07:09

Focal length and camera-to-subject distance completely describe magnification. Why would I list two quantities when I could list one equivalent quantity?

Because magnification is a dependent variable, and focal length and subject distances are independent variables. Focal length was held constant in the original stated "problem", so summarizing that value into another dependent variable hides its significance.

One might as well state that depth of field depends only on blurriness. Indeed.

Of course, we spend a lot of time talking about what appears to be sharp, and sometimes forget to talk about the appearance of what is not sharp. A difference in depth of field that has little apparent effect on what is sharp, based on a particular scene, might have a profound impact on the appearance of the unfocused elements. And changing the angle of view and subject distance changes those relationships necessarily.

Thus, the original question is unrealistic (a necessary element in all good bar bets--and many "good" academic discussions), and I expanded the distance between the formats to highlight that fact. The 300mm normal focal length for 8x10 that would easily make a full-face portrait on that format at a distance of two feet (for a magnification at the negative of 1:2) would come in a telephoto package for small format that could probably not even be focused at that distance. But if you put it on an extension tube and managed to focus it, the magnification would still be 1:2, and the picture would include only the subject's nose. The depth of field would be the same, when comparing the small-format image to the portion of the large-format image covering the same portion of the subject. Enlarging them to the same nose size would not affect that relationship, but enlarging them to the same print size would, though other faults might mask the effects.

Doing the things that most people do involves a lot of assumptions: Enlargement ratio is greater with small format, but not usually to the point where the print sizes are the same; focal lengths will be different; apertures will be different as the photographer will usually have more choices with small-format lens at the wide end, especially if selective focus is the objective. Subject distance might be the same, because that affects the fundamental composition. Or, it might not--photographers often set up big cameras at heights convenient for their tripod, while small-format camera positions are limited by the ability of the photographer to hold the camera up to his eye--a different set of constraints. In practice, one must use much wider apertures with their small-format camera to achieve the same "look" as when using a large-format camera. But that wasn't the bar bet, and assuredly was not in question by the combatants in that bet.

Rick "who always states his assumptions" Denney

Brian Ellis

9-Apr-2010, 08:06

The same thing always happens when people talk about depth of field, which is that some people talk about what happens in the camera and others interject the print. There are many variables that affect perceived depth of field in the print, size of print/degree of enlargement being the most obvious. Saying that depth of field depends on the degree of magnification is, as someone (Rick I believe) said earlier, like saying it depends on the degree of blurriness. Yep, that's right. But everything that happens with the print starts with the circles of confusion in the negative. All of the variables that follow are determined by the plane of focus and the size of the circles of confusion on either side of it. Those are fixed and invariable once the photograph is made and are determined by three things and three things only - aperture, focal length, subject distance. That's my story and I'm sticking with it. : - )

Nathan Potter

9-Apr-2010, 09:17

This thread reminds me of my professor wife having debates with her academic colleagues. They will essentially all say the same thing, each using slightly different nomenclature, and will argue about it ad infinitum.

It happens so often that I coined the term, "getting academiced" whenever she (or anyone else) amends or corrects a statement by another by saying the exact same thing with only key changes in vocabulary or phrasing.

Ha, do I know about that! This discussion is in the wrong language, english, when it should be in the language of math. The math relationships can be fiddled around with but are inviolable, at least in the frame of classical electrodynamics. The depth of field is just derived from the geometry of image formation. So the near limit Dn and the far limit Df can be determined by:

Df=uf^2/(f^2-NCu) and Dn=uf^2/(f^2+NCu)

Combine these two relationships and we have:

DOF (depth of field) = 2u^2NC/f^2 where:

u = the object conjugate (object distance)
N = f/number
f = lens focal length
C = desired circle of confusion

One can play with and plot these relationships as one wishes but the truth is implicit in the equations. Of course the above relates only to the situation where m (magnification factor) is not a variable (it ain't in the formula and is < say 0.1).

Nate Potter, Austin TX.

Paul Kierstead

9-Apr-2010, 11:03

Truth is not in the equations, because the equations don't tell us what we want. Your math tells us that given those four factors, this is what the DoF is. But, the problem is that we don't necessarily agree on the values of those factors, because it is the statement of the problem that is typically at fault, not the solution. You are presenting math, but no one disputes the math.

For example, you present it as a function of CoC. But what determines CoC? Well, for example, perception. Also, print size. Also, resolution (there would be no point in setting CoC lower then resolution; some would argue it should be resolution, but I think that is silly as an absolute). Given two different formats, do we assume the same print size? Do we assume the same expectation of "in focus"? If, in one system, maximum resolution in the exact plane of focus is higher then the other system, do we assume the CoC is still the same, or should it be some function of the maximum resolution?

More to the point, you present lens focal length as a factor, but what determines focal length? The desired composition. The desired composition, in this context, is probably a factor of both working distance (u, which affects visual persepective) and field of view (lets say v). So, given two different formats, do we hold u the same, or just d, or just v? Clearly, if we modify u and then let d fall where it may in order to maintain f, we will get one outcome. If we keep f the same and don't modify u, we must either crop (in which case it isn't a different format) or we must let v change, in which case we don't have the same composition, making the comparison somewhat moot.

I could go on. Math does not reveal truth, it is just one of the tools needed.

Jeff Conrad

9-Apr-2010, 22:20

Nate, I think something got lost in your final expression for DoF.

That said, I find it hard to understand why the waters need be muddied by saying that the math doesn't tell us what because our assumptions may differ. So state the assumptions; some of us may use different values for CoC because of differences in final-image size or viewing conditions. But if we're making a comparison, it would make no sense to use different values for final-image CoC. Once we fix that (whatever the value), the comparison then becomes fairly simple.

It's easier if the DoF is given in terms of magnification; for distances small in comparison with the hyperfocal distance (the common assumption often not stated), we have

DoF ~ 2Nc (m + 1) / m^2

Here m is the magnification of the captured image; c is determined by the enlargement of the captured image to the final image. For magnification small in comparision with unity (another common assumption often not stated), this simplifies to

DoF ~ 2Nc / m^2

The comparison is then just a matter of the ratio of these values for two different formats; the values of the variables depend on what is assumed. The specifics are covered in the Wikipedia article I linked above, so I won't repeat them here. This isn't a new idea; Stroebel discussed it in 1976 (and perhaps earlier; the 3rd ed. is the version I have).

It should be noted that the comparisons hold only within the distance restrictions stated. But it should suffice to answer the original question. And that answer depends on what's assumed, which isn't completely stated in the original post.

Darin Boville

10-Apr-2010, 02:22

Paul (just above) starts to bring this back into the realm of pictorial photography--thanks for that.

Yet with almost two dozen posts on this issue, on a forum populated almost entirely by photographers, all we see is equations and arguments over equations, and not a photograph to be seen.

Wouldn't the point be made clearly enough with a polaroid from a 4x5 and a shot from a DSLR, both with he same focal length/f-stop, both with the same (as close as possible) composition and focus plane, and both prints/scans at the same size?

Why are photographers so hesitant to reach for their cameras?

--Darin

Jeff Conrad

10-Apr-2010, 05:48

A strictly photographic comparison might illustrate the point (and would be a helpful addition), but it wouldn't prove it. And it would be more work, especially when one has the equations at hand; to cover all the possibilities discussed, a series of comparison photographs would be needed. And the conditions would need to be carefully chosen to provide a good illustration of the effects--potentially trial and error unless one already has a rough idea on what the conditions should be from understanding of the principles involved. If I were writing a book on the topic, the effort might be justified. But offhand, I wouldn't otherwise have much motivation for making such a set of images.

What's interesting about many of the posts, including Paul's, is that most of us seem to agree that the answer depends on what we assume, yet we don't seem to see that we're saying the same thing.

I'm even surprised that the behavior is in dispute; I've always thought it was common knowledge that for the "same picture," the DoF decreases with increasing format size. Perhaps the OP added a twist by requiring the same focal length, rather than the more common "same picture." Perhaps many of us are also accustomed to taking the DoF as a given and adjusting the f-number to achieve it.

For the same final-image size, final-image sharpness criterion (i.e., CoC), viewing conditions, and f-number,

If the subject distance is the same and the focal length is changed to maintain the same angle of view, DoF is inversely proportional to format size. To me, this is the "same picture."

If the focal length is the same and the subject distance is changed to maintain the same field of view, DoF is inversely proportional to format size. But the perspective and the angle of view change, so to me, this isn't the "same picture."

If the focal length and the subject distance are the same for both formats, DoF is directly proportional to format size because the smaller format is given more enlargement. The perspective is the same, but the field of view is different, so it isn't even close to the same picture, and the comparison would seem silly.

So I guess the original question could be answered by choosing one of the possibilities above. There are a few other possibilities, such as using the same f-number and subject distance and giving each image the same enlargement--the DoF then would be the same for both formats. But this would also seem a silly comparison.

Incidentally, nearly all formulas for DoF take the "subject distance" to be the object conjugate, i.e., the distance from the lens's front nodal plane to the subject, rather than the film-to-subject distance, and most of the posters seem to follow this convention. So there's no ambiguity about the magnification.

Dan Fromm

10-Apr-2010, 06:42

Jeff Conrad wrote:

"Incidentally, nearly all formulas for DoF take the "subject distance" to be the object conjugate, i.e., the distance from the lens's front nodal plane to the subject, rather than the film-to-subject distance, and most of the posters seem to follow this convention. So there's no ambiguity about the magnification."

Stuff and nonsense. As long as subject distance as you define it is > 2f, there are two possible magnifications. If the distance is long, only one is practical but there are always two. And when the distance is relatively short, which of the two is selected has practical significance.

ret wisner

10-Apr-2010, 10:35

its all relative

Darin Boville

10-Apr-2010, 11:28

Dan Fromm

10-Apr-2010, 11:40

Darin,

I think we're in two different universes. You want to see the pictures. For others, me included, the math is enough.

I used to give a program on shooting fish in aquaria to aquarium societies. The DoF question came up a lot, often in the context of "which focal length gives the most DoF?"

To nail it once and for all, I made a series of Kodachromes of the same subject shot with the same lighting at 1:4 with a 55/2.8 MicroNikkor, a 105/2.8 MicroNikkor, a 200/4 MicroNikkor (the old AI/AIS version, not the AF one), and a Questar 700. I used 1:4 because that's as high as the Questar would go on its own mount. The 'chromes were indistinguishable, couldn't be matched to the lens that took them. I can't show you the slides, I'm know which closet they're in but not where in it and they've never been scanned.

I don't know whether you want to believe me, but I did what I say and the results were as I report them. One could do the same thing with lenses for a larger format.

Cheers,

Dan

Bill_1856

10-Apr-2010, 14:08

What a bunch of BS. In fact, Depth of Field is an imaginary quantity -- it can be anything you want. There is NOT a distance around the principal focus plane where everything is sharp, only varying amounts of fuzzyness. The "depth of field" is just defined as how much fuzzyness of the image you are willing to tolerate.
The Circle of Confusion is a number which will tell you mathmatically just how fuzzy the image will appear (given the distance, aperture, focal length, etc), but it is an arbitrary number based on how much fuzz you are willing to put up with.
So the answer to your original question is: "It all depends...."

Struan Gray

10-Apr-2010, 14:26

Let's not forget that a 90 mm lens for 35 mm will be a telephoto, and pupillary magnification comes into DOF too ... :-)

I have four 50-ish lenses:

47 mm Super Angulon f5.6
50 mm f2.8 Distagon for Hasselblad
50 mm f1.7 Pentax-A
50 mm f2.8 Pentax-110

The 110 lens is a telephoto, and has the best depth of field. I use it on 4x5 all the time - as a loupe.

PS: This means I agree with Drew's friend, the D3 shot will have more depth of field, iff you use lenses intended for the formats.

W K Longcor

10-Apr-2010, 14:29

Paul Kierstead

10-Apr-2010, 16:03

I presume you are assuming no print is made? Or, if a print is being made, that the print also varies in size?

Jeff Conrad

10-Apr-2010, 16:38

"Stuff and nonsense. As long as subject distance as you define it is > 2f, there are two possible magnifications."

With all due respect, I don't know where you're coming from. For every given film-to-subject distance > 4f, there indeed are two possible magnifications: the object conjugate can be either the long or short distance. But fixing the object conjugate (the "distance from the front of the camera," of sorts) fixes the magnification.

Jeff Conrad

10-Apr-2010, 16:49

Keeping it simple --AND to answer the original question --does dof change with FORMAT? NO--- Example: set up an 8x10 camera with a 300mm lens. Focus on a subject 10 ft away. Make an exposure on 8x10 film. remove camera back and substitute a 5x7 reducing back and make a new exposure. Do the same with a 4x5 reducing back. now -- again using a roll film adaptor.
You now have four different FORMATS -- all with identical dof.:eek:

If the final images (prints or whatever) are contact printed or given the same enlargement, all the formats will indeed have the same DoF. But that's not the usual basis for comparison, especially 35 mm vs. 4x5. Again, by varying the conditions, it's possible to come up with almost any desired answer.

Jeff Conrad

10-Apr-2010, 17:04

PS: This means I agree with Drew's friend, the D3 shot will have more depth of field, iff you use lenses intended for the formats.

Struan, I think you and I agree in principle (this was my first bullet above). But I think you're being a bit free with Drew's money if the bet indeed specified the same focal length for both formats; even then, Drew's friend could have been right if the subject distances were adjusted to maintain the same field of view (my second bullet). But it's not clear that this was stipulated, so absent other knowledge, I'd be inclined to call it a draw. Especially if it were my money ...

The pupillary magnification indeed enters into DoF (we had a long discussion about this five or six years ago), but the lens would need to be quite asymmetrical and the magnification approaching unity (or greater) for the effect to be significant. At closeup distances, the assumption used for the normal format comparison is no longer valid. Of course, the distances weren't specified, so perhaps the common statement that DoF is inversely proportional to format did not apply. Again, perhaps a draw by default.

Struan Gray

10-Apr-2010, 23:54

Jeff, you are being altogether too reasonable - and practical. In the bars I know, the whole point of a bet like this is to invoke as much obscure knowledge and arbitrary precision as is needed to crush the opposition :-)

Next up: travel alarm clocks with corrections for special *and* general relativity.

Jeff Conrad

11-Apr-2010, 00:42

Struan, I'd have had a better attitude with a couple more pints ...

Struan Gray

11-Apr-2010, 04:37

Not at Swedish prices.

Martin Miksch

11-Apr-2010, 07:02

CoC is not a fixed value, but defined by "agreement" about what looks acceptable sharp. So in 8x10 you have much larger CoCs than in 35mm, becouse you need less enlargment.
Regards
Martin

rdenney

12-Apr-2010, 04:36

>>A strictly photographic comparison might illustrate the point (and would be a helpful addition), but it wouldn't prove it.<<

A *strictly* photographic comparison? Oh, I think it's o.k. to use a few words along with the pictures :) But nothing here seems to be "proving" anything.

When people ask questions on a forum, they are not looking for a photographic comparison. They are looking for proof, and the proof has been provided here whether people recognise it as such or not.

There is nothing preventing any of us from conducting tests to see for ourselves the effects. A question on the forum would not be necessary for the OP or anyone else to do that.

Rick "leaving it as an exercise for the reader" Denney

rdenney

12-Apr-2010, 04:57

Let's not forget that a 90 mm lens for 35 mm will be a telephoto, and pupillary magnification comes into DOF too ... :-)

I have four 50-ish lenses:...

Your pupils must magnify better than mine.

In response to a similar argument (more on that in a minute) along these lines on Photography on the Net a few years ago, I did actually perform the experiment, with the following lenses:

- 47mm Super Angulon
- 50mm Flektogon
- 50mm Canon (for both 35mm and APS-C)

I didn't have a Pentax 110 (hanging his head in shame). But I did have an APS-C Canon DSLR.

The exposures were made on 6x9, 6x6, 35mm, and APS-C, respectively. The principle argument in that case was that perspective was not a function of focal length with respect to format, but rather a function of camera position. That's not likely to be an item of debate on this forum. But in addition to illustrating that, I also was able to show that a 24x36mm frame from the middle of all three images were basically identical in appearance, including their depth of field. (I will add that some lenses show smaller airy disks off-focus than others, all else equal, and this must be a function of their design. It's not surprising, given that some lenses show different bokeh effects than others, too.)

Back to the argument. What argument? The only argument I've seen so far is how one should prove the point. Nobody seems to disagree with that point, though there are lots of arguments about the terms used. It would appear to me that Mr. Bedo now has the means at his disposal to turn a bar bet into a full-fledged brawl, complete with airborne chairs and broken crockery.

Rick "expecting a report" Denney

rdenney

12-Apr-2010, 05:07

CoC is not a fixed value, but defined by "agreement" about what looks acceptable sharp. So in 8x10 you have much larger CoCs than in 35mm, becouse you need less enlargment.

[raises his hand...]In the telescope world, the fuzzy spot produced by all optics, and generally constrained by diffraction in good optics, is called the airy disk. I have always considered this the term appropriate to the fuzzy-spot effect, but maybe it does not apply to spots made fuzzy because of being out of focus. The circle of confusion seems to me the standard by which the dimension of the airy disk is evaluated. At some point, it is small enough so that the detail it portrays is no longer confused. But that is an assumption on my part. Right? Wrong?

Rick "who divides all DOFMaster CofC values by four to convert their 8x10 print assumption to 16x20" Denney

Struan Gray

12-Apr-2010, 13:44

Your pupils must magnify better than mine.

In theory at least.

Any differences will show up at large magnifications, 1:1 and above. In principle, you should see the change in DOF clearly by reversing the lens, but you can find yourself fighting the asymmetry another way as one of the object/film conjugates moves inside the front element. Aberrations complicate the issue more than somewhat too.

The Pentax 110 50 is an oddball. The pupillary magnification of the seperate lens is around 2, a little more if you put an aperture where it was supposed to be in the original system, which had an combined aperture and shutter inside the camera body. The right way round, it makes a great loupe (or wide-field telescope eyepiece), but I'm too lazy to expose fine-grain film with it to settle someone else's bar bet. I trust the maths.

At least I know to capitalise "Airy" :-)

Struan

Mike Tobias

12-Apr-2010, 14:03

Don't think anyone's posted this yet so here it is. To date the best examination of this issue I know of:

http://www.naturfotograf.com/D3/D3_rev06.html

Short answer (if you don't want to read it), maybe there's a difference, but it's so small as to be almost unmeasurable.

Mike

Jeff Conrad

12-Apr-2010, 14:36

A bit off the original topic, but ...

Rick, the Airy disk is a different animal from the defocus blur spot; it has different causes as well as different appearance. The Airy pattern arises from diffraction, and consists of light and dark concentric rings. The defocus blur spot arises from, well, defocus, and is more uniform in appearance. Defocus and diffraction combine to reduce sharpness, but for what we've been talking about here, diffraction (and the Airy disk) shouldn't be much of an issue, save perhaps each lens's smallest aperture. Or at closeup distances, where diffraction does become significant, and where the simple model breaks down anyway. Or unless it is essential to test some even more obscure knowledge (or perhaps technobabble one's way out of a lost bet. Probably works better with pints than with decaf.).

Just out of curiosity, why do you multiply DOFMaster CoCs by four for 16x20? Two would seem to do (unless you always pull out your loupe).

Darin Boville

12-Apr-2010, 15:22

Jeff Conrad

12-Apr-2010, 15:47

Short answer (if you don't want to read it), maybe there's a difference, but it's so small as to be almost unmeasurable.

Ummm ... the difference is night and day when you compare the images at f/8 in the second example. For the FX-format image at f/8 and the DX-format image at f/5.6, the DoFs are about the same, as also would be expected. Yet again, it depends on the conditions under which the comparison is made. The usual assumptions for the "same picture" case (which he examines in the "same FOV" example) again are

Same subject distance (i.e., object conjugate) for both formats.
Subject distance significantly less than hyperfocal for both format.
Magnification significantly less than unity.
Same final-image size for both formats
Same f-number for both formats.

At magnifications of 0.13 and 0.2, the third condition is violated. Even so, with the final assumption (comparing the results at f/8), there's clearly much greater DoF for the DX-format image.

I think this also illustrates some problems with strictly photographic comparisons:

The subject distance is too small, so what's being compared isn't within the model's valid range.
Though it's obvious that f/8 DX image has considerably greater DoF than the f/8 FX image, it's not immediately apparent precisely how they differ (it's not exactly 1.5 because the magnification in the numerator isn't negligible).

It seems to me that the results clearly demonstrate the validity of the DOF models we've been discussing, and which have been well understood for years. It boggles the mind that people continue to attempt to rediscover this stuff.

Paul Kierstead

12-Apr-2010, 15:55

Jeff Conrad

12-Apr-2010, 16:37

What baffles me is that it is absolutely, 100%, beat you over the head obvious that, in practice, the DoF when using LF is vastly less then for 35mm.

So one would think. But it seems pretty obvious that the FX format has a lot less DoF than the DX format in the cited article, yet the author reaches the opposite conclusion ...

Dan Fromm

13-Apr-2010, 03:19

Paul, my flower pictures don't agree with you. But then, I went up in format to be able to get more of the main subject's setting in the frame, not to shoot at a higher magnification than with 35 mm.

Jeff Conrad

13-Apr-2010, 03:28

Dan, things change a lot with closeups--the differences between formats aren't as great, so what you're saying is exactly what I would expect. Again, the common rule of thumb relies on quite a few assumptions; if they aren't met, all bets are off.

So really, you and Paul are probably both right--you're just assuming different conditions.

Jack Dahlgren

13-Apr-2010, 06:53

What baffles me is that it is absolutely, 100%, beat you over the head obvious that, in practice, the DoF when using LF is vastly less then for 35mm. Anyone who has shot any number of pictures at all will easily observe this. I suppose one could argue that LF has the same DoF, it just requires a lot more light. Now that is in practice. One can mathematically masturbate till the cows to come home to show all sorts of other conclusions (some of them even interesting), but it is all just that; masturbation. If you are concerned about outcomes, then it is clear.

Of course, when it comes to bar-bets, it pretty much depends on wankery, or else it wouldn't be a bet would it?

Try this thought experiment - or do it for real. Instead of a sheet of film, tape a 35mm frame of film in a film holder and expose it. The image is exactly the same without regard to the format of the film right? Suppose that small image is exactly what I want to capture. Now how do I do that while keeping everything else the same? Well the only thing I can do is move in much closer so the subject fills more of the frame.

The reason it seems that dof is less with LF "in practice" is that you are closer to the subject when using it. My small format 135mm lens doesn't even focus closer than a couple of meters, yet that is a typical shooting distance for a 135mm on large format. See what sort of depth of field you get with a 300 mm lens on small format a few feet away from the subject.

Paul Kierstead

13-Apr-2010, 07:16

Jack, the whole point of in practice is that it isn't an experiment or contrived example. And it isn't picking a lens length on two formats and comparing them (unless, I suppose, your job is lens testing or something).

I'll give you this thought experiment: Got out and take some head and shoulders portraits on 35mm. Use appropiate lenses to get a natural look. Now do the same with 4x5: Go shoot some head and shoulder portraits with a natural look. How is the DoF going to compare? Now that is in practice.

sgelb

13-Apr-2010, 07:54

Ok: This is a little like a bar-bet though it came up over coffee. We were discussing portraiture. I shoot LF and my friend (a professional wedding photographer) shoots with a top-of-the-line DSLR, (Nikon D-3).

My question: For the same focal length and same aperture; does DOF change with format? My thought was that 90mm at 5.6 would give nose-eyes-ears sharp and blur the background regardless of how large the film was.

My friend says that DOF will be shallower in LF for the same focal length and aperture.

For me to be right, the image size must also be the same . . .right?

Waitress: Two more over here please . . .thats right—De-Caff.

the larger the format, the shallower the DOF.

f/2.8 on 4x5 is like .95 or so on 35mm

Robert Hughes

13-Apr-2010, 09:02

the larger the format, the shallower the DOF.

f/2.8 on 4x5 is like .95 or so on 35mm
Wrong - you're mixing too many variables. A 150mm lens on 4x5 is exactly same DOF as 150mm on 35mm format, and with a 150mm lens, f/5.6, DOF is exactly the same for any format.

rdenney

13-Apr-2010, 09:20

Just out of curiosity, why do you multiply DOFMaster CoCs by four for 16x20? Two would seem to do (unless you always pull out your loupe).

Because 8 X 4 = 16, silly. (That was a joke; you can laugh. Of course, two is what I meant.)

Actually, the Zeiss article mentioned in another thread just filled my last hour, and provided a little more clarity on the terms, though I also found that my understanding was sufficient. It really should be that "circle of confusion" describes the diameter of the blurry spot, and not to the standard by which it is evaluated. That standard should probably be called the "critical circle of confusion diameter".

The Airy disk confusion seems forgivable: I always evaluated telescope performance the conventional way by looking at the diffraction rings with the telescope slightly defocused inside and outside, when doing a star test.

Rick "thankful not to have to revise his bokeh article after reading that Zeiss white paper" Denney

rdenney

13-Apr-2010, 09:25

Speaking of the Zeiss white paper linked in another thread, I thought I would link it here, despite that providing authoritative descriptions has little relevance to a bar bet. But here it is anyway:

Zeiss white paper on depth of field, depth of focus, and bokeh. (http://www.zeiss.com/C12567A8003B8B6F/EmbedTitelIntern/CLN_35_Bokeh_EN/$File/CLN35_Bokeh_en.pdf)

Rick "who has added this to his library" Denney

Darin Boville

13-Apr-2010, 10:35

Wrong - you're mixing too many variables. A 150mm lens on 4x5 is exactly same DOF as 150mm on 35mm format, and with a 150mm lens, f/5.6, DOF is exactly the same for any format.

Of course, you might want to make prints from the negs...

--Darin

Jeff Conrad

13-Apr-2010, 18:09

Because 8 X 4 = 16, silly. (That was a joke; you can laugh. Of course, two is what I meant.)

That standard should probably be called the "critical circle of confusion diameter".

Circle of confusion wears many hats, and terminology varies among sources. Blur spot due to defocus is arguably the best choice, because it isn't always circular (it's the shape of the aperture, sometimes distorted if off axis), and with tilt or swing, even a circular blur spot is elliptical. It's not possible to separate the blur spot due to defocus from that due to diffraction, so even this is a bit of a misnomer.

Most sources I've seen formally use something to the effect of acceptable circle of confusion diameter, sometimes omitting diameter, but generally soon fall back to just circle of confusion because the strictly correct term gets pretty unwieldy. Formally, I usually use something like defocus blur spot, but quickly go to something like CoC because anything more is just too much to read repeatedly. As long as it's made clear what's meant, there usually isn't a problem.

That said, it's sometimes also important to indicate whether the CoC (or whatever) applies to the initial image or the final image. It's usually clear from context, but I've seen some pretty heated arguments that derived simply from two different interpretations. Especially after a few pints ...

Darin Boville

13-Apr-2010, 18:25

>>whether the CoC (or whatever) applies to the initial image or the final image.<<

Exactly. Perhaps it is just my reading skills but that distinction has not been very clear at times throughout this thread...

In any event, to my mind it is the final image that should be the "standard" for judging depth or field or CoC since, presumably, we are talking about pictorial photography...I've seen very few negatives on display...

--Darin

Jeff Conrad

13-Apr-2010, 20:56

... to my mind it is the final image that should be the "standard" for judging depth or field or CoC ...

Darin, I think almost all of us completely agree with you. And the value for initial-image CoC in the DoF formulas derives from that assumption. If C is the final-image CoC and E is the enlargement, the initial-image CoC is just C/E. It would be easy enough to express DoF directly in terms of final-image CoC, but for whatever reason, it isn't commonly done.

At least to me, it wouldn't make much sense to base a comparison on different sized final images, so the initial-image CoC should be in direct proportion to the format size.

Though I mentioned "same final-image sharpness" in my first post, I see that I neglected it in my bullet list above; that list should read

Same subject distance (i.e., object conjugate) for both formats
Subject distance significantly less than hyperfocal for each format
Magnification significantly less than unity for each format
Same final-image size for both formats
Same final-image sharpness criterion for both formats
Same viewing distance for both final images
Same f-number for both formats

So I guess we really do need to be careful to state everything we ass-u-me.

Emmanuel BIGLER

14-Apr-2010, 03:35

Hello all !
I'm returning to this discussion after being silent for a while : the Zeiss white PaPer by Dr. Nasse has an interesting section about cheating with FTM curves and the consequences on apparent dof.
It explains what several users of digital cameras have observed, i.e. a "razor-sharp" transition between in-focus and out-of-focus planes in a digital image ; this confirms that digital sensors perform like film ... except for digital enhancements done secretly before delivering the image file to the photographer !

and [OFF-TOPIC] From Struan and friends fascinated by luxury time-keeping instruments :
Next up: travel alarm clocks with corrections for special *and* general relativity.

I am writing this post while attending our European Time and Frequency annual forum in the Netherlands (elevation : zero metres, sometimes less than that)
Recent research reports indicate an ultimate relative stability of 10^{-17} for some kind of trapped-ion atomic clocks ; clock comparisons have been made recently at the level of 10^{-16}.
The corrections for altitude due to general relativity are in the order of 10^{-14} for 1000 metres of elevation, so coming back from the Netherlands to the Jura mountains I'll have to reset my miniature wrist-atomic-watch.
So next step is to implement a compact trapped-ion atomic clock in Struan's alarm-clock.
At a Swedish retail price of course.:D

Emmanuel BIGLER

14-Apr-2010, 04:28

* Same subject distance (i.e., object conjugate) for both formats
* ....
* Same ...........for both formats

An additional item is required to Jeff's list :
* same [public or secret or whatever], or no digital pre-processing prior to the final print comparison ! ;)

Struan Gray

14-Apr-2010, 05:38

So next step is to implement a compact trapped-ion atomic clock in Struan's alarm-clock.

The GPS in my mobile sort of qualifies. They had the sense to put the atomic clock in orbit, safely out of range of my daughter and her trusty set of crosshead Wihas.

I used to say I wanted a clock corrected for special relativity, but the cunning chaps and chapesses at NIST (http://tf.nist.gov/ofm/smallclock/CSAC.html) forced me to up the ante.

I realised that I am actually in a position to do a real comparison, since I recently bought a package of LF stuff which included a Nikkor 360 T, so in theory I could see how it stacks up against my 360 Symmar. I suspect the Nikkor's relatively poor performance at close focus will swamp any DOF differences. If anyone wants to send me a 240 or 420 mm tessar-type process lens I would be willing to compare it to my symmetrical plasmats and dialytes at 1:1 on my laser table. I'm not holding my breath though.

rdenney

14-Apr-2010, 06:34

That said, it's sometimes also important to indicate whether the CoC (or whatever) applies to the initial image or the final image. It's usually clear from context, but I've seen some pretty heated arguments that derived simply from two different interpretations. Especially after a few pints ...

I did mention it, but in a way that complicates the issue.

At one extreme, we might limit ourselves to contact prints. A 24x36mm frame of film used in a 35mm camera with a 300mm lens will render nearly* identical depth of field to an 8x10 sheet exposed with a 300mm plasmat, as determined by comparing the 35mm frame to a 24x36mm fragment of the 8x10 sheet.

(* discounting differences caused by lens design, of course. As the Zeiss paper showed us, the circle of confusion depends on the pupil diameter--the exit pupil in the case of depth of focus and the entrance pupil in the case of depth of field. And the pupil diameters depend on the positions of the nodal points, which depend on the design of the lens.)

At the other extreme, we compare prints of the same size.

In practice, however, we rarely do either. I might make a 12x18" print from my Canon 5D, or a 16x20 print from my Pentax 6x7. Those are a 12.7x enlargement and a 7.3x enlargement, respectively. I might also limit 4x5 to 16x20, simply because that's as big as my printer will go, in which case that print is a 4x enlargement. Thus, in practice, it seems unrealistic to compare the formats at the same degree of enlargement, or even at the same print size.

Also, we do not compare images made with a 300mm lens on 24x36 to an image made with a 300mm lens on 8x10, for the simple reason that they don't make the same image anyway. Either they don't include the same subject, or they require us to move the subject or camera which changes the subject distance (and, hence, the depth of field) and the perspective. Such comparisons are the stuff of bar bets, but not of practical photography.

But there are extremes in that comparison, too. We might, on the one hand, insist that each format uses a lens that produces the same image in terms of perspective and framing. So, if we use a 300mm lens on 8x10, we must compare it to a 40mm-ish lens on 35mm, without accounting for the different aspect ratio. The other extreme is to follow the given condition in the bar bet, which is to use the same focal length in two different formats. In practice, though, we don't necessarily do either. For example, I have on many occasions made portraits using a 35mm camera (or even an APS-C camera) and a 180mm or 200mm lens. I frequently use a 180mm/2.8 Nikkor on my Canon 5D, for example. To attain the same magnification on an 8x10 camera, however, one would need a 1400mm lens. That seems pretty unlikely. The practical fact is that photographers make focal length decisions not to match the perspective and framing of another format, but to take advantage of the format they are using, and the lenses they own.

The full-face portrait I used as an example is probably the situation providing the closest comparison. One might quite reasonably make a facial portrait filling half the frame using a 300mm lens on 8x10 or a 50mm lens on 35mm, and to get the same degree of background blur*, the latter would need an f-stop many stops wider, no matter whether comparing the negatives or prints of the same size. That is a practical fact.

I suspect that is why, in practice, we can say large formats have less depth of field than small formats. We enlarge the small formats more, but not as much more as we would if making the same size prints, so neither the comparison at the same degree of enlargement nor the comparison at the same print size is perfect. And we don't always choose lenses that provide the same subject distance and framing, simply because we have different artistic objectives and different lenses at hand when using different formats.

Rick "realizing that bar bets and practicality are often mutually exclusive" Denney

Brian Ellis

14-Apr-2010, 08:02

Darin, I think almost all of us completely agree with you. And the value for initial-image CoC in the DoF formulas derives from that assumption. If C is the final-image CoC and E is the enlargement, the initial-image CoC is just C/E. It would be easy enough to express DoF directly in terms of final-image CoC, but for whatever reason, it isn't commonly done.

At least to me, it wouldn't make much sense to base a comparison on different sized final images, so the initial-image CoC should be in direct proportion to the format size.

Though I mentioned "same final-image sharpness" in my first post, I see that I neglected it in my bullet list above; that list should read

Same subject distance (i.e., object conjugate) for both formats
Subject distance significantly less than hyperfocal for each format
Magnification significantly less than unity for each format
Same final-image size for both formats
Same final-image sharpness criterion for both formats
Same viewing distance for both final images
Same f-number for both formats

So I guess we really do need to be careful to state everything we ass-u-me.

You're confusing depth of field with acceptable sharpness in a print. They aren't the same thing. Depth of field refers (or should refer) to the size of the circles of confusion on either side of the plane of focus in the film or sensor. Size of the circles of confusion is affected by three factors and three factors only - subject distance, aperture, and focal length. Perceived sharpness in a print is a different matter. It's affected by the size of circles of confusion in the film or sensor but also by other things, all of which are almost infinitely variable, such as degree of enlargement, viewing distance, lighting conditions, visual acuity, etc.

The short and IMHO accurate answer to the original question is that film format alone has no effect on depth of field, all other things affecting depth of field (aperture, focal length, and subject distance) remaining equal. But whether one finds a print acceptably sharp depends in part on depth of field but also on other factors such as print magnification, viewing distance, etc.

Paul Kierstead

14-Apr-2010, 08:32

Depth of field refers (or should refer) to the size of the circles of confusion on either side of the plane of focus in the film or sensor.

Why should it refer to the sensor, not the presentation medium?

BetterSense

14-Apr-2010, 09:38

Depth of field refers (or should refer) to the size of the circles of confusion on either side of the plane of focus in the film or sensor.

Actually, that would be depth of focus. Depth of field applies in the object plane.

Jeff Conrad

14-Apr-2010, 18:54

Depth of field refers (or should refer) to the size of the circles of confusion on either side of the plane of focus in the film or sensor.

One man's opinion. Hard to say whether it's right or wrong, but it's certainly at marked odds with convention in the context of DoF. It seems to me that by definition, DoF is the range of object distances for which objects appear sharp, so perception is inherent in the concept. A comparison could be made by directly viewing images captured on film (I'm not sure how to do it with an electronic image ...), but it would be a fun exercise if both images were captured on color negative film ... and it would still depend on viewing conditions.

If the quoted statement is correct, then every camera and lens manufacturer in every format has it wrong. Can that really be the case? It seems to me that we're debating something about which there really hasn't been much dispute since before any of us were born.

Size of the circles of confusion is affected by three factors and three factors only - subject distance, aperture, and focal length.

I don't think there's any disagreement that for the same object distance and f-number, all lenses of the same focal length give the same magnification in the initial image. Without enlargement, both have the same DoF. Again, they still rely on viewing conditions and human perception.

Perceived sharpness in a print is a different matter. It's affected by the size of circles of confusion in the film or sensor but also by other things, all of which are almost infinitely variable, such as degree of enlargement, viewing distance, lighting conditions, visual acuity, etc.

Of course--that's why all the assumptions need to be stated. We could view both images in the dark; does an image have DoF if no one can see it?

Jeff Conrad

14-Apr-2010, 19:29

I suspect that is why, in practice, we can say large formats have less depth of field than small formats. We enlarge the small formats more, but not as much more as we would if making the same size prints, so neither the comparison at the same degree of enlargement nor the comparison at the same print size is perfect.

We could argue forever about the "right" way to make a comparison, and I don't think there's really an answer. But if we state the conditions of the comparison, we can probably answer the initial question. And as the conditions change, so will the answer.

But there are extremes in that comparison, too. We might, on the one hand, insist that each format uses a lens that produces the same image in terms of perspective and framing.

I'm probably an extremist ... I think any comparison such as we're talking about is necessarily contrived. But if I wanted a particular image, I'd probably choose the same camera position regardless of format. And if I wanted a particular composition, I'd probably choose a lens in whatever format that was closest to giving the desired framing. And I'd probably want the same extent of sharpness in any format, so if possible, I'd adjust the f-number to provide the desired DoF regardless of format. In most cases, I'd need an f-number that was in direct proportion to the format size, which is the same as saying that for the same f-number, the DoF is in inverse proportion to the format size.

Would I make the same size final image from a 35 mm capture as from 4x5? In many cases, probably not, so the same-size final image is arguably the most contrived of the conditions.

Were I forced to use the same-focal-length lens for different formats, I might, in some cases, adjust the subject distance to get the same subject framing. But the picture would be quite different, and I'm not even sure a meaningful comparison would be possible. Objectively, if we limited our comparison to the size of the final-image defocus blur spots, we'd find DoF in inverse proportion to the format size.

Jeff Conrad

14-Apr-2010, 19:45

Actually, that would be depth of focus. Depth of field applies in the object plane.

Depth of field and depth of focus are slightly different concepts. The former normally refers to the range of object distances that are imaged with acceptable sharpness on a fixed image plane; the latter normally refers to the range of image distances over which an object at a fixed distance is imaged with acceptable sharpness (e.g., film buckling or misfocus).

The two phenomena are approximately conjugate; however, the depth of focus is symmetrical about the image plane, while this is only approximately true for the image distances corresponding to the limits of depth of field.

Darin Boville

14-Apr-2010, 22:53

does an image have DoF if no one can see it?

If no one sees an image not only does it not have depth-of-field it doesn't even exist as an image! That fact that the image obviously continues to exist in the dark, and that the inks/dyes/grains of silver are still rearranged in such a way to cause "depth-of-field" even with the lights out, merely reveals that the image is at all times being viewed by a power higher than that of man.

Depth-of-field illustrates--indeed demonstrates--the existence of God.

--Darin

John Bowen

15-Apr-2010, 02:17

Whenever I'm at a bar and switch formats, say from lite beer to single malt scotch, my DoF decreases and my Circle of Confusion increases....

Sorry, I couldn't help myself and besides this thread needed some hummor....

GPS

15-Apr-2010, 02:38

If no one sees an image not only does it not have depth-of-field it doesn't even exist as an image! ...

--Darin

Not true! If you bring a group of blind people to a gallery, none of them sees any image. Yet the images do not stop existing, they are still there with their depth of field... ;)

Jeff Conrad

15-Apr-2010, 02:48

Do you use the same glass for both formats?

GPS

15-Apr-2010, 03:30

I think that's clear enough that the bet concerns the same glass of beer for both formats.

rdenney

15-Apr-2010, 06:12

I'm probably an extremist ... I think any comparison such as we're talking about is necessarily contrived.

Which was my point.

But if I wanted a particular image, I'd probably choose the same camera position regardless of format.

Hmmm. When I walk around with a small camera, I use the camera as a viewing card, looking for compositions that please me. When I see one, I push the button. That "viewing card" has a much wider range of options available to me than does my large-format camera.

When using a large-format camera, I walk around without the camera, using my mind as the viewing card. Given that I'm photographing using the large-format camera, the range of possibilities is already built into my thinking, so I automatically edit out image opportunities for which I have no lens.

Also, I have different objectives when I use large-format. Images I would reject with smaller cameras because of their lack of image-management tools I can attempt with a view camera. And the image-management tools and format characteristics of the view camera lead me to explore images that highlight what those tools and characteristics provide. I have made images on small cameras that would be impossible on a large camera, and vice-versa. Knowing those differences informs our image-selection process whether we do it on purpose or not.

I suspect few select their subjects, compositions, and tonal visualizations without any consideration for the equipment at hand.

If I had a particular composition in mind before leaving home, then I would bring the equipment that would help me realize that visualization. That is not always the big camera, of course. But that happens rarely with me--maybe I see an image or a fragment of an image while driving by or while exploring, and come back later. But even then, the details of my visualization will often migrate to the capabilities of the equipment I brought.

But the real point is that we tend to set extreme examples for our comparisons: Either no enlargement or enlargement to the same print size. I suspect most do something in between, which just emphasizes how much those extreme cases are contrived.

Assumptions are a model of the conditions, and, as they say (maybe not in bars), all models are false, but some are useful.

Rick "allowing alternative explanations" Denney

Jeff Conrad

15-Apr-2010, 07:18

I usually using a real viewing card regardless of format so that sloth doesn't have undue influence on the camera position. Ansel would be proud of me ...

But the real point is that we tend to set extreme examples for our comparisons: Either no enlargement or enlargement to the same print size. I suspect most do something in between, which just emphasizes how much those extreme cases are contrived.

To be honest, I don't spend a lot of time thinking about comparisons. I usually carry only one format at a time, and I get whatever DoF I get (as I said, I usually change the f-number to suit). But if a comparison, however contrived, is to be made, we need to state the conditions; they don't necessarily need to be my "same picture" criterion. That's really all this discussion is about.

Maybe the real answer is to go easy on the single malts and avoid the bets ... or at least avoid the bets.