Is it me ?

[Stephen Lewis]

I was always under the impression that the only factors affecting depth of field (DOF) were the focal length of the lens, the aperture used, and the point of focus distance. DOF is independent of format size, film or digital.

Imagine my suprise then when, this weekend, I opened a landscape photography supplement from one of the big magazines to find an article on (DOF) which showed two tables for hyperfocal distance, one for full size sensors and one for APS-C sized sensors.

For a 50mm lens at F8 one table gives a hyperfocal distance of 55ft, the other 35ft. If I fit a digital back with a 645 sensor to my 5x4 camera, why should that change the DOF characteristics of all my lenses? Am I missing something :confused:

[Dan Fromm]

Yes, what matters isn't distance but magnification. The APS-C sensor is smaller, so works at lower magnification than a full size sensor.

Thinking in terms of focused distance is safe only when the format is always the same.

[Stephen Lewis]

Are you saying that the DOF is affected by the format size? Surely changing from a 5x4 sheet to a 6x9 roll film back doesn't affect the characteristics of a lens which has already been fitted, set and focussed? The smaller format simply results in a smaller area of the lens' overall coverage being used?

[Joanna Carter]

Are you saying that the DOF is affected by the format size? Surely changing from a 5x4 sheet to a 6x9 roll film back doesn't affect the characteristics of a lens which has already been fitted, set and focussed? The smaller format simply results in a smaller area of the lens' overall coverage being used?
Not at all. It's all down to something known as the "Circle of Confusion". See this site (http://www.dofmaster.com/dofjs.html) for an online calculator that will demonstrate the differences CoC can cause.

[Joanna Carter]

Not at all. It's all down to something known as the "Circle of Confusion". See this site (http://www.dofmaster.com/dofjs.html) for an online calculator that will demonstrate the differences CoC can cause.
Although, I would say that simply putting a different back on the same LF lens wouldn't cause the same problem as a 35mm camera with a 90mm lens compared to a 4x5 camera with a 90mm lens. As Dan says, it's all down to magnification; a 90mm lens on a 35mm camera will not give you the same magnification as the same focal length as on a 4x5. Compare the size of an SLR lens with that of an LF lens.

[Dan Fromm]

Are you saying that the DOF is affected by the format size? Surely changing from a 5x4 sheet to a 6x9 roll film back doesn't affect the characteristics of a lens which has already been fitted, set and focussed? The smaller format simply results in a smaller area of the lens' overall coverage being used?

Yes, and no.

Yes is the image captured is identical, except for size, on the two formats. In this case, magnification is lower on the smaller format. This is not the situation you described.

No if the magnification is the same. This is the situation you described.

You have the advantage on us. You have the article. Would you revisit it and tell us more about what's in it? It might, great stress might, explain which of the situations I've just described the tables cover.

Cheers,

Dan

[Ken Lee]

Adding Circle of Confusion to the calculation (see the web site to which Joanna has provided a link), takes into consideration the fact that different sized film or sensors, require differing amounts of enlargement to make a print of the same size.

What might be an insignificant degree of blur in a contact print, or a mere 2X degree of enlargement, begins to look genuinely out of focus, at greater magnification.

For example, since 8x10 film requires 1/2 the enlargement of 4x5, the calculator allows a correspondingly larger circle of confusion, twice as large. Since 6x7 film requires roughly twice the enlargement of 4x5 film, the allowable circle of confusion used in the formula is roughly 1/2 that given for 4x5... etc.

[Martin Miksch]

At least we reached the circles of confusion, I hope all will found out again^^

[Aender Brepsom]

Surely changing from a 5x4 sheet to a 6x9 roll film back doesn't affect the characteristics of a lens which has already been fitted, set and focussed? The smaller format simply results in a smaller area of the lens' overall coverage being used?

Yes, that is exactly how I understand it and that is why I agree with you first post:


I was always under the impression that the only factors affecting depth of field (DOF) were the focal length of the lens, the aperture used, and the point of focus distance. DOF is independent of format size, film or digital.

Of course, the final image will not be the same (might be heavily cropped), but that was not your question if I understood you well. If you try to reproduce the exact same picture on different media (4x5, 6x9, 35mm, full frame DSLR vs crop DSLR, etc.), then things will be different as magnification, different focal lengths and other factors will come in.

If you cut a 4x5 sheet to 6x9 format, it will be identical to the 6x9 shot taken with the same lens and settings.

[Ken Lee]

"At least we reached the circles of confusion, I hope all will found out again^^"

Dante would be proud !

[Ron Marshall]

Are you saying that the DOF is affected by the format size? Surely changing from a 5x4 sheet to a 6x9 roll film back doesn't affect the characteristics of a lens which has already been fitted, set and focussed? The smaller format simply results in a smaller area of the lens' overall coverage being used?

In this case there is not change in DOF; the magnification is not changed by cropping the ground glass image using the 6x9 back.

But, in order to have the same framing (and perspective) on 6x9 that you have with 4x5 you must use a shorter focal length on the 6x9; shorter focal length, less magnification, greater DOF.

[Paul Kierstead]

Are you saying that the DOF is affected by the format size? Surely changing from a 5x4 sheet to a 6x9 roll film back doesn't affect the characteristics of a lens which has already been fitted, set and focussed? The smaller format simply results in a smaller area of the lens' overall coverage being used?

Ah, you've nailed it perfectly, and obtained a useless result, since the two pictures won't be the same content, now will they? How about not being extraordinarily pedantic for a moment, and consider the desired result (a picture of some specific composition from a specific shooting position).

Sheesh, will this dead horse every stop twitching?

[Nathan Potter]

Language is as bad as mathematics in its ability to confuse simplicity. (Photographic proverb).

Nate Potter, Austin TX.

[Vaughn]

Language is as bad as mathematics in its ability to confuse simplicity. (Photographic proverb).Nate Potter, Austin TX.

I prefer the old Monty Python line...

"A wink is as good as a nod to a blind man." :D

Vaughn

[aduncanson]

Proving once again that nothing is as contentious or as confusing among photographers as Depth of Field.

It is Ken Lee that nailed it. Depth of field calculations require assumptions, if not knowledge, about how much magnification will take place from the negative to the print, as well as the viewing distance for the print. Focal length, aperture, object distance and focus distance will uniquely determine the size of the Circle of Confusion, but the negative that gets more enlarging, requires a smaller CoC to look as sharp in the print.

I really, really find it very unhelpful to discuss the differences in DoF between formats, if you fail to scale the focal length to the negative height or width so that essentially identical compositions can be compared.

[Ben Syverson]

The Cliff's Notes on DOF is:

- If you have a lens that represents a fixed horizontal angle of view, say 45°, DOF will increase inversely to sensor size. The larger the sensor, the less DOF for the same HFOV.

- If you keep the lens and aperture the same, DOF does not change for different sensor sizes. While a 90mm on 4x5 will give you a radically wider photo than the same lens on 35mm, a 35mm-sized crop of the 4x5 will look like what you expect a 90mm shot on 35 to look like. We say the 90mm on 4x5 has "more" DOF than on 35 because you're not likely to print it to the same magnification as the 35mm crop. Printing smaller increases apparent DOF.

[Stephen Lewis]

So it's me then!

[Joanna Carter]

So it's me then!
Well, sort of :D

[Alan Davenport]

Are you saying that the DOF is affected by the format size? Surely changing from a 5x4 sheet to a 6x9 roll film back doesn't affect the characteristics of a lens which has already been fitted, set and focussed?

YES! Depth of field is absolutely dependent on the format size!

DOF calculations are based on the amount of blur that can be present in the image but will still be perceived as "in focus" by the viewer. This is obviously a subjective thing, and it requires us to begin with certain assumptions.

The first assumption is a standard print. For reasons lost in antiquity, the standard print was chosen to be an 8x10, viewed from something like 10 inches away. Or maybe it was 12 inches. Let's go with 12; it's not critical to this discussion, just work with me. The rest of the numbers I'm using are correct.

On that 8x10 print, viewed from 12 inches away, is a blur circle. It turns out that people with good vision (or whatever kind of vision they chose to define DOF) will perceive a blur 0.2mm across as a single point. That's because persons with this variety of good vision can just separate two points that are 0.2mm apart. Put the two points closer than that and our "standard person" will see only one point. So our second assumption is that we are allowed a circle of confusion (blur circle) on the print of 0.2mm.

Now we have a standard for "in focus," which can be used to calculate depth of field. It's based on a standard print (8x10) which is viewed at a standard distance (12 inches) by a person with standard vision (can resolve 0.2mm at 12 inches.)

[Disclaimer: some people have different vision, make different sized prints and view them at different distances. That's nice. Nobody cares. DOF is a very subjective thing, but we're talking about standards right now.]

On the film, you can have a circle of confusion which when enlarged to the 8x10 print will be at or below the limit of 0.2mm. For 4x5 film, which only needs 2X enlargement for an 8x10, you are allowed a 0.1mm circle of confusion on the film without being perceived as "out of focus." Change to 120 film and a 6cmX7cm format, and you now have to enlarge 3.6X to get an 8x10 print, so you can only have a 0.05mm circle of confusion on the film.

Let's take an example, using a 150mm lens at f/8, focused at 10 meters. With 4x5 film, allowing a 0.1mm CoC, our DOF will extend from 7.4m to 15.4m, for a total DOF of just about 8 meters. Leaving the lens alone and changing to 6x7 format rollfilm, the DOF will go from 8.5m to 12.1m, a total DOF of 3.6 meters. Thus the depth of field characteristics have changed, even though we changed nothing except the size of the film.

I'll stop now.

[edit: I used f/calc to get the numbers for CoC examples. Apparently f/calc assumes 4X enlargement for 6x7 film to get an 8x10 print.]

[aduncanson]

...

The first assumption is a standard print. For reasons lost in antiquity, the standard print was chosen to be an 8x10, viewed from something like 10 inches away.

...


10 inches is taken as a standard close focus capability of person with good vision. At that distance an 8x10 print pretty much fills the field of vision that a person can view without losing the corners to degraded peripheral vision. So, while one can clearly make a larger print and view it from closer, or even view it with a magnifier (If that is your standard, then you need to apply a smaller circle of confusion.) but for most people to do so they are no longer looking at the photograph as a whole.

[Jim Michael]

There are many companies in the video industry dedicated to the production of DoF adapters for video cameras with small (ca. 1/3" to 2/3") sensors to mimic the shallow(er) DoF of 35mm cinema cameras. They achieve the effect by filming the image produced by a lens intended for 35mm cameras projected onto a moving ground glass (spinning or vibrating to as to eliminate the ground glass artifacts).

[Ken Lee]

Let's take an example, using a 150mm lens at f/8, focused at 10 meters. With 4x5 film, allowing a 0.1mm CoC, our DOF will extend from 7.4m to 15.4m, for a total DOF of just about 8 meters. Leaving the lens alone and changing to 6x7 format rollfilm, the DOF will go from 8.5m to 12.1m, a total DOF of 3.6 meters. Thus the depth of field characteristics have changed, even though we changed nothing except the size of the film.

So (to put a fine point on your fine explanation) the depth of field for the smaller film size, is actually smaller - because of its need for greater enlargement.

However, because the film size is smaller, that same 150mm lens now gives us a narrower angle of view, and we have to step back twice as far from the subject, to get the same basic photo.

Now what happens to depth of field ?

[Nathan Potter]

I was always under the impression that the only factors affecting depth of field (DOF) were the focal length of the lens, the aperture used, and the point of focus distance. DOF is independent of format size, film or digital.

Imagine my suprise then when, this weekend, I opened a landscape photography supplement from one of the big magazines to find an article on (DOF) which showed two tables for hyperfocal distance, one for full size sensors and one for APS-C sized sensors.

For a 50mm lens at F8 one table gives a hyperfocal distance of 55ft, the other 35ft. If I fit a digital back with a 645 sensor to my 5x4 camera, why should that change the DOF characteristics of all my lenses? Am I missing something :confused:

The first part of your statement is entirely correct for any given lens and any given format size. Since Depth Of Field (DOFld) =2u^2NC/f^2 where u = subject distance, N = f/no., C = circle of confusion, f = focal length. The size of the film or image sensor dosen't matter. Discussion of the media size and magnification is irrelevant.

However the original article, as vaguely described by you, is (I'm guessing) normalizing the taking media (two different sized sensors with different pixel spacings) to a change in the COC that would be required to produce images of equal resolution and format. This would be an entirely legitimate but subtle elaboration involving C, above, as a new variable. I think IMHO this is what Dan Fromm was saying in his "yes" and "no" answer but he'll correct me if I'm wrong. :) :)

Nate Potter, Austin TX.

[venchka]

Let's take an example, using a 150mm lens at f/8, focused at 10 meters. With 4x5 film, allowing a 0.1mm CoC, our DOF will extend from 7.4m to 15.4m, for a total DOF of just about 8 meters. Leaving the lens alone and changing to 6x7 format rollfilm, the DOF will go from 8.5m to 12.1m, a total DOF of 3.6 meters. Thus the depth of field characteristics have changed, even though we changed nothing except the size of the film.

So (to put a fine point on your fine explanation) the depth of field for the smaller film size, is actually smaller - because of its need for greater enlargement.

However, because the film size is smaller, that same 150mm lens now gives us a narrower angle of view, and we have to step back twice as far from the subject, to get the same basic photo.

Now what happens to depth of field ?

Is it a black lens or a chrome lens? Does the lens finish match the camera body finish?

:D Sorry. I couldn't resist. Foolishness I learned at the Rangefinder Forum where folks are forever agonizing over mismatched lens & body finishes.

[Leonard Evens]

You have to start off by realizing that everything you thought you knew about depth of field is probably wrong. It is definitely not an inherent property of the lens. A good source which discusses some of the issues you have raised is at
http://photo.net/learn/optics/dofdigital/
You might also look at Jeff Conrad's article on lfphoto.info
http://www.largeformatphotography.info/articles/IntroToDoF.pdf
if you can tolerate the formulas

The formulas for depth of field have several different forms. In one form, you need to specify the focal length of the lens, the f-number, the acceptable circle of confusion (coc), and the distance of the subject to the lens. Alternately, you need to specify the focal length, the f-number, the circle of confusion, and the scale of reproduction or magnification (i.e., the ratio of image size to subject size in the exact plane of focus.) But however you do it, you need to specify four parameters.

The circle of confusion is a measure of what you consider an acceptable lack of sharpness. It is the diameter of the largest disc that you can't distinguish from a point. But what you consider to be acceptable will depend on what you are looking at and from where. If we concentrate on the final print, you will be willing to accept a larger coc if you are further from the print. Also, some people may be able to see better than others and they would choose a smaller coc for the same size print viewed from the same distance.

Since the final print is usually enlarged from the image formed in the camera, you need to take the format into account. This is done by dividing the coc acceptable in the print by the degree of enlargement to make the print. For example, an 8 x 10 print viewed at 10-12 inches might accommodate a coc of 0.2 mm. But you would reduce that to 0.2/2 = 0.1 mm at the level of the film for 4 x 5 which must be enlarged 2 times to make the print . For 35 mm, if you plan to enlarge 8 times, you would have to reduce the coc at the film level to 0.2/8 = 0.025 mm. That means, the same person looking at the same size final print from the same viewing point, would predict very different depth of field for the same focal length and the same f-number for 4 x 5 than for 35 mm.

Format size also enters into the choice of focal length in case you want to maintain the same angle of view. Thus, a 150 mm lens on 4 x 5 has the same angle of view as a 300 mm lens on 8 x 10. The shape of the format can also play a role. 35 mm has an aspect ratio of 3:2 which is different from the 5:4 aspect ratio for 4 x5 or 8 x 10. That means it isn't entirely clear which angles should be compared. Depending on what you choose, ratios of focal length of 3.33, 3.5, and 4 each can be justified.

For subjects which are relatively close to the lens, but not necessarily so close to be considered close-ups, for a fixed format, the depth of field depends essentially only on the scale of reproduction and is essentially independent of focal length. Often portraits are within this range. But, if you change the format, the change in acceptable coc still yields a significant change in DOF, even if you were to keep the magnification at the level of the film the same.change of depth of field

The independence of focal length discussed above only holds for relatively close subjects, For distant subjects, the focal length plays an important role. People often get confused about this, and since much large format photography is of distant subjects, it is important to keep it in mind.

[Ken Lee]

Bravo !

The calculator to which Joanna provided a link, uses 4 parameters, and a quick glance at the underlying Javascript, suggests that the author has used the formulas to which you refer.

It works nicely, and gives results which support a rule of thumb which you shared with us before: Given the same subject distance, when switching from 4x5 to 8x10, if we double the length of the lens to get the same angle of view, we will need to stop down the longer (8x10) lens by 2 stops, to get the same DOF we got on 4x5.

It also shows how depth of field changes when you take the same basic picture, and change lens + format accordingly, to get the same view. Maintaining a constant subject distance of 10 feet, and keeping the lens stopped down to f/16

6x7 film + 100mm lens: DOF = 6.16 feet
4x5 film + 200mm lens: DOF = 2.31 feet
5x7 film + 300mm lens: DOF = 1.47 feet
8x10 film + 400mm lens: DOF = 1.06 feet

[walter23]

Are you saying that the DOF is affected by the format size? Surely changing from a 5x4 sheet to a 6x9 roll film back doesn't affect the characteristics of a lens which has already been fitted, set and focussed? The smaller format simply results in a smaller area of the lens' overall coverage being used?

If you enlarge the 6x9 negative and the 4x5 negative to the same final print size, the 6x9 will have a smaller DOF under otherwise equal conditions (same lens, focus, etc), because of the greater enlargement. If you're just looking at contact prints at the same distance the 6x9 negative is just a straight crop of 4x5, and so DOF will be the same; it's no different from cutting out that section of a 4x5 negative and looking at it instead.

Think of it this way; a detail that looks sharp on a 4x5 negative enlarged to 8x10 might look blurry on a 6x9 crop of that negative which was also enlarged to 8x10. For a more extreme example, imagine the details on a 35mm contact print, which look sharp at the unmagnified contact print "thumbnail", but become obviously blurry once enlarged. Same idea. Enlargement can blur things that look sharp at smaller sizes.

This gets confusing though because if you're considering equivalent *field of view* instead of focal length (e.g. a 100mm lens on 6x9 vs. a 300mm lens on 8x10, both fairly "normal" focal lengths), you're switching lenses, and so the decreased depth of field from a longer lens counteracts the magnification difference so that in the end larger formats have less DOF. Same sorts of things happen if you keep the same lens on but step back or forward to frame up your main subject the same way; the larger formats will have less DOF in the end, despite the counteracting tendency of the different magnification.

[Alan Davenport]

So (to put a fine point on your fine explanation) the depth of field for the smaller film size, is actually smaller - because of its need for greater enlargement.

However, because the film size is smaller, that same 150mm lens now gives us a narrower angle of view, and we have to step back twice as far from the subject, to get the same basic photo.

Now what happens to depth of field ?

You're getting us into the second lecture in the series, Ken!

In my original example (150mm lens, 10m subject distance) the 4x5 format has a field of view of roughly 8.3m X 6.7m. To get that same field in 35mm (it won't match exactly because the aspect ratio is different) we have to move back to about 42 meters.

At 42 meters, on 35mm film and still at f/8, our 150mm lens' DOF will go from ~31m out to ~67m, for a total depth of 36 meters. Obviously not the same.

To get comparable depth of field at differing subject distance, we have to vary the aperture such that the aperture will have the same angular dimension as seen from the subject position. Since we've moved 4X farther away, the aperture diameter needs to be 4X as large if we want the same DOF. Using f/2 in our new setup (35mm @ 42 meters) will give us about the same DOF that we got using f/8 on 4x5 @ 10 meters.

[aduncanson]

Since we've moved 4X farther away, the aperture diameter needs to be 4X as large if we want the same DOF. Using f/2 in our new setup (35mm @ 42 meters) will give us about the same DOF that we got using f/8 on 4x5 @ 10 meters.

But in no way does that produce the same photo! Only under the very loosest of standards would they even be comparable. In the 4x5 photo, the far focus is 39% greater than the near focus and the more distant subject will be significantly de-emphasized. In the 35mm photo the far focus is only 9% further than the near focus and the subjects will be relatively equal in scale. The relationship to out-of-focus foreground and background objects is even more radically changed.

Why is this comparison of more than hypothetical interest?

[Ken Lee]

"Why is this comparison of more than hypothetical interest?"

If you have ever marveled to see a pencil appear to be broken, when placed in a glass of water, you will appreciate that although DOF is a basic element of photography, it's not an easy one to understand, because there is more to Optics, you might say, than meets the eye.

It's fascinating because we get surprising results.

[Steve M Hostetter]

all I can remember about hyperfocal distance is that a 50mm lens on a Mamyia RZ (6x7cm) is roughly 12 feet ,,,, so if I am using a wide angle on any format I tend to use the 12' distance as a starting point..
in practice you focus on an image and then tilt and refocus and maybe tilt some more and refocus till you see a totally sharp image on the GG and just have to say well I must of used the hyperfocal distance somewhere in there ...making it more a ridged camera concern
and as you know a 210mm isn't a wide angle untill you mount it to an 8x10 or bigger format

[Alan Davenport]

Why is this comparison of more than hypothetical interest?

Only because that's the direction the thread went...

[Wally]

I prefer the old Monty Python line...

"A wink is as good as a nod to a blind man." :D

Vaughn

It must be me. I thought it was a blind horse.:confused:

[Kirk Gittings]

In this case there is not change in DOF; the magnification is not changed by cropping the ground glass image using the 6x9 back.

But, in order to have the same framing (and perspective) on 6x9 that you have with 4x5 you must use a shorter focal length on the 6x9; shorter focal length, less magnification, greater DOF.

Bang on. This was part of the reason I switched to 6x9 roll film from 4x5 for commercial architectural photography a few years back. More DoF at a given aperture meant less strobe fill for interiors etc. The DoF I was getting on 4x5 at F16 with a 90mm, I could get at F11 (more or less) on a 65mm on 6x9. That one stop difference meant allot in the real world of shooting.

[bglick]

Here is a relatively simple cheat sheet for DOF analysis between formats which takes all the variables into consideration....


1) Change format size, change lens fl.....

same DOF exists, when you have the same:


a) recording media (same film type or same density digital sensor)
b) composure (half the format diag, = half the fl)
c) camera to subject distance (same angular view of lenses)
d) resolution on same final print size
(double the format diag. = half cc diam.)


Result ..... 2x format diag, double f stop = 2 stops slower shutter speed for the same DOF. So the penalty for equal DOF in larger format is 2 stops of shutter speed.

or, as Kirk corectly mentioned above, very painful when using strobes, as strobes are dependent on f stop only. Hence why LF is certainly not a flash friendly format.



2) Change lens fl, change camera to subject distance,

same DOF exists, when same:


a) format size (film size or digital sensor size, same density)
b) composure (half fl, half subject to camera distance)
c) focus point (but not focus distance)
d) cc (due to same size format)

Result is..... half fl = 1/2 f stop (2 stops faster).


It's interesting to note, and rarely mentioned, when using larger formats, there is a loss of resolution at the point of exact focus vs. the smaller format. DOF assumes only the max. size cc, not the min. sizes.


However, these are general rules. The reality is, nothing is carved in stone with modern optics. DOF formula was created in the late 1800's IIRC.... there was very few optical variables to contend with back in those days. With modern optics, many things have changed, including the use of aspheres, which can radically change the expected DOF output and sharpness at point of focus. Also, different lens designs react differently to DOF. A math formula that lumps all lenses into a single formula is over generalization today. Aberations vary greatly with each lens type... and at each f stop and even at different focus distances. This is evidenced by viewing MTF data of lenses at different focus distances.

Also, the DOF formula is deceiving.... the reason is, you simply plug in variables, and the formula pumps out answers. The Optical chain is not that simple in the real world. For example, you enter a given cc..... what knowledge do you have that cc can actually be recorded on film / sensor? The formula does not prevent you from using rediculous values that are not achievable, as in real world recording, the MTF values of the lens and the film play a critical roll in what's possible. Example, use DOF formula and shoot with color neg film, then the same with B&W high rez film, compare the results.... the color neg film will probably fall short of your expectations, the B&W film will far exceed your anticipated outcome.

cc is also effected by shutter vibrations, wind, film flatness, etc. etc. too many variables to ever expect an over simplified formula to provide accurate result.

So buyer beware on the DOF formula subject....

I have tested a lot of lenses. I have noticed with newer aspheric designs, the resolution at the point of exact focus on 35mm lenses, will often not vary at f2.8 through f11. This was unheard of 10+ years ago. The aspheres change all the rules as we know them, specially the rules (DOF formula) created in the 1800's.

So IMO, its OK to use these general rules of thumb to get close to what you want to achieve, but often today, it requires testing to be sure you meet your desired objectives...

[GPS]

...

However, these are general rules. The reality is, nothing is carved in stone with modern optics. DOF formula was created in the late 1800's IIRC.... there was very few optical variables to contend with back in those days. With modern optics, many things have changed, including the use of aspheres, which can radically change the expected DOF output and sharpness at point of focus. Also, different lens designs react differently to DOF. A math formula that lumps all lenses into a single formula is over generalization today. Aberations vary greatly with each lens type... and at each f stop and even at different focus distances. This is evidenced by viewing MTF data of lenses at different focus distances.

Also, the DOF formula is deceiving.... the reason is, you simply plug in variables, and the formula pumps out answers. The Optical chain is not that simple in the real world. For example, you enter a given cc..... what knowledge do you have that cc can actually be recorded on film / sensor? The formula does not prevent you from using rediculous values that are not achievable, as in real world recording, the MTF values of the lens and the film play a critical roll in what's possible. Example, use DOF formula and shoot with color neg film, then the same with B&W high rez film, compare the results.... the color neg film will probably fall short of your expectations, the B&W film will far exceed your anticipated outcome.

cc is also effected by shutter vibrations, wind, film flatness, etc. etc. too many variables to ever expect an over simplified formula to provide accurate result.

So buyer beware on the DOF formula subject....

I have tested a lot of lenses. I have noticed with newer aspheric designs, the resolution at the point of exact focus on 35mm lenses, will often not vary at f2.8 through f11. This was unheard of 10+ years ago. The aspheres change all the rules as we know them, specially the rules (DOF formula) created in the 1800's.

So IMO, its OK to use these general rules of thumb to get close to what you want to achieve, but often today, it requires testing to be sure you meet your desired objectives...

This is the same type of reasoning as to say that the equations for area calculations used to cover a swimming pool with a piece of material are old and out of touch with reality- the materials changed a lot, a modern plastic cover is not as soft and stretching as a piece of cotton cloth etc...

[bglick]

> This is the same type of reasoning as to say that the equations for area calculations used to cover a swimming pool with a piece of material are old and out of touch with reality-



Sorry, I don't see the parallels....unless fabrics have changed substantially over the past 100 years, like optics have.


I cited specific examples of why DOF can run astray.... such as entering cc values not attainable on the recorded media, the use of aspherical elements which changes the typical spherical pattern of lens performance we are accustom to seeing and different types of lens designs. I would think addressing these specific issues would be more productive than making a swimming pool analogy.... but its a public forum....


When the DOF equation was created, lenses were mostly doublets and film was at its infancy at best. DOF was based on the infamous "thin lens equation" which represents the most simplest of optical design forms, mainly singlets and doublets... a far cry from todays lens designs.


The purpose of the DOF formula is determine the amount of resolution which can be recorded at the near/far distances. However, the amount of resolution recorded is a function of the optical performance of a lens and the performance characteristics of the recording media. Neither of these are taken into consideration in the DOF formula. And over the past 100 years, the improvements in these two variables have been remarkable.


However, since the DOF formulas premise is based on a given focus distance and a given fl lens.... the general premise still applies today, i.e. the point of exact focus will always be the sharpest recorded area of the image, with the near/far being less sharp.... (till you get into very small apt., where the two level out) DOF formula tries to define "how much less sharp" the near/ far points will record. Yet, there is no information on the recording media and lens performance. Therefore, as per the DOF formula, an 1880's vintage lens, and a modern lens should perform identical, when using the same fl, format and focus distance. Draw your own conclusions....

[GPS]

[QUOTE=bglick;472757...

Sorry, I don't see the parallels....unless fabrics have changed substantially over the past 100 years, like optics have.


....[/QUOTE]

Haven't they?? Check modern sports cloths, to say just one example!

What you try to describe as some old fashioned optical equations is rather a simple difference between the theoretical values and the practical values. The fact that a car doesn't have the same practical consumption as the theoretically calculated consumption doesn't prove that the theoretical equations are "too old" - just that there is a difference in the correct theoretical values and the practical values.
A square area calculation equation is not incorrect because the material you cover the area with is less stretching today than it was 100 years ago...