DOF question 35mm vs. 4x5

[stradibarrius]

Is is correct that LF lenses and 35mm lense of the same focal length and set to the same f/stop would produce images with different DOF's??? If possible can someone give an "Explanation for Dummies" type answer as to what the difference and why?

[Heroique]

I'll try to keep this very simple for starters...

If you're using, say, a 100mm lens on a 35mm camera, and a 100mm lens on a 4x5 camera, each with the same f/stop, and at the same distance from the subject, it's the same DOF.

Of course, if you want the same framing for the two formats, you'd move the 35mm camera back from the subject, and that would provide greater DOF. Or you might move the 4x5 camera closer, which would provide less DOF, or "greater isolation," as some people say.

[Mark Woods]

Excellent description!

[duff photographer]

Is is correct that LF lenses and 35mm lense of the same focal length and set to the same f/stop would produce images with different DOF's??? If possible can someone give an "Explanation for Dummies" type answer as to what the difference and why?

I'll have a go but I stand to be shot down for being too simplistic.

Put simply, in the case you give the depth of field will actually be the same. If you took a 35mm (24x36mm) segment of that 4"x5", the depth of field will be the same. Another way of looking at is if I used a 35mm camera with a 35mm lens I would get exactly the same DOF as a 35mm digital back (covering the same format) shooting through a large format lens of the same focal length.

It is the larger format that effectively reduces the depth of field because it is covering more of the scene at the same focal length. Conversely a lens on an APS format will cover a more limited area of the scene so will give the impression of a greater DOF.

However, the difference in DOF shows itself best when comparing the same image coverage/angle of view between the systems. To get the same image coverage shown by a 35mm DSLR on a 4"x5" you'd have to use a lens which has a focal length of, in this case, about 3.3 times that of the 35mm format. Therefore a lens with a focal length of 50mm in 35mm format will need to be a 165mm for 4"x5" format. In other words, the 165mm large format lens on a 4"x5" will have the same angle of view as the 50mm on a 35mm DSLR (full frame).

The DOF of a 165mm lens is inherently less than that on a 50mm lens so you would then see a real world difference in DOF when comparing the images of the same dimensions and angle of view.

I think I've got that right. :-\

DP

[stradibarrius]

Thanks that makes perfect sense!!!


I'll try to keep this very simple for starters...

If you're using, say, a 100mm lens on a 35mm camera, and a 100mm lens on a 4x5 camera, each with the same f/stop, and at the same distance from the subject, it's the same DOF.

Of course, if you want the same framing for the two formats, you'd move the 35mm camera back from the subject, and that would provide greater DOF. Or you might move the 4x5 camera closer, which would provide less DOF, or "greater isolation," as some people say.

[BetterSense]

It's confusing because f-stops are a terrible unit of measure.

For a given composition, the only thing that determines DOF is the diameter of the lens aperture. If lens apertures were expressed in something sensible, like say mm or inches, we wouldn't have this confusion. An aperture of 5mm will give the same DOF for any camera format. Very simple. Obvious.

A long time ago, someone decided it was a great idea to express lens apertures as the diamters of the aperture divided by the focal length. So now, when I set up a still-life composition, I have to use 4 different f-stop settings depending whether I'm using a 35mm, 6x7, 4x5, or 8x10, to get the same aperture and thus DOF.

[Leigh]

Basic point...
The lens has no idea what film is behind it.

So obviously the DoF at the image plane is a fixed value.

Take a shot with a particular lens (300mm??) on 8x10 film.
Now cut a 24mmx36mm rectangle out of that film.
Has the DoF of that portion of the subject changed?

- Leigh

[Dan Fromm]

It's confusing because f-stops are a terrible unit of measure.

For a given composition, the only thing that determines DOF is the diameter of the lens aperture. If lens apertures were expressed in something sensible, like say mm or inches, we wouldn't have this confusion. An aperture of 5mm will give the same DOF for any camera format. Very simple. Obvious.

A long time ago, someone decided it was a great idea to express lens apertures as the diamters of the aperture divided by the focal length. So now, when I set up a still-life composition, I have to use 4 different f-stop settings depending whether I'm using a 35mm, 6x7, 4x5, or 8x10, to get the same aperture and thus DOF.

Very interesting. Why do you believe all this, BS?

Ror everyone else, which DoF concept are you talking about? DoF on film or in the final print? And are you talking about the same final print when you say that a short lens gives more DoF than a long lens?

[Leigh]

It's confusing because f-stops are a terrible unit of measure.
Why?

f/stops progress at a ratio of 1.414:1.

Since the f/stop (aperture diameter) determines the area of the pupil, each stop differs from its neighbors
by a factor of 2 (in square millimeters or square inches or square whatever).

- Leigh

[Patrick13]

Go any deeper down this rabbit hole and you'll have to introduce "Circle of Confusion" into this beginner's introduction and that may not be appropriate.

f/stops are a great unit, without conversion an f/stop of F11 on a 50mm lens yields exactly the same light transmission as an f/stop of F11 on a 300mm lens. This is perfect for the end user taking a picture and well worth the hidden engineering to make it happen via diameters and much less important than DOF changes and all that when taking a picture.

[Jac@stafford.net]

It's confusing because f-stops are a terrible unit of measure.

For a given composition, the only thing that determines DOF is the diameter of the lens aperture. If lens apertures were expressed in something sensible, like say mm or inches, we wouldn't have this confusion. An aperture of 5mm will give the same DOF for any camera format. Very simple. Obvious.

Another example of Texas science. Good luck with that.
.

[Vaughn]

For me the biggest difference is that with 4x5 camera movements I can place the plane of focus (and its DoF on each side of it) in the scene more creatively than with 35mm or MF.

[Jeff Conrad]

It's confusing because f-stops are a terrible unit of measure.s

It all depends on the primary objective. Lens apertures are marked in f-numbers to facilitate exposure, which arguably is a sine qua non. This comes at the expense of simple DoF equivalence. I’m not sure this is such a big deal, though, because unlike exposure, the aperture required to get the desired DoF is context dependent, determined not only by the distance between the near and far objects but also on the camera distance from either—move the camera, and you need to redetermine the aperture to get the desired sharpness.


For a given composition, the only thing that determines DOF is the diameter of the lens aperture.

No disagreement. But implicit in “given composition” are several assumptions that are often overlooked:

Same camera position
Same framing (i.e., focal length is approximately proportional to format size)

Perhaps not directly related to composition, but equally implicit:

Same final-image size (i.e., enlargement is inversely proportional to format size)
Same viewing conditions (normal assumption: 8x10 final image viewed at 250 mm, with proportionally greater distance for a larger image)
Same resolution of the imaging media
Same final-image sharpness criteria (OK, arguably a no-brainer ...)

To me, this is the only sensible way to compare images taken with different formats—but many of the seemingly endless discussions on this topic—including this thread—don’t state these assumptions.


So now, when I set up a still-life composition, I have to use 4 different f-stop settings depending whether I'm using a 35mm, 6x7, 4x5, or 8x10, to get the same aperture and thus DOF.

True enough, though I’m trying to remember the last time I was in this situation. Of course, as always, YMMV. But would it be any easier if the aperture markings were in absolute diameters and you had to deal with four different exposures?

[BetterSense]

It all depends on the primary objective
I see what you did there. Bravo!


f/stops are a great unit, without conversion an f/stop of F11 on a 50mm lens yields exactly the same light transmission as an f/stop of F11 on a 300mm lens. This is perfect for the end user taking a picture and well worth the hidden engineering to make it happen via diameters and much less important than DOF changes and all that when taking a picture.

This is an opinion I don't share. It's logical that the same composition and aperture diameter gives less exposure when it's spread over an 8x10 piece of film vs. half-frame 35mm. Fundamentally, I have no reason to expect a given aperture to result in the same exposure time for vastly different camera formats, but gee, wouldn't it be nice if the same aperture number resulted it the same, know, aperture, so that I could count on the image physics being the same. I have camera with auto-exposure...have you ever seen a camera with auto-DOF?


True enough, though I’m trying to remember the last time I was in this situation.
Maybe you don't shoot still-lifes. Maybe you don't switch formats often. Personally, I would prefer working in real apertures. Then I could internally remember that a typical head-and-shoulders portrait needs an aperture of 3mm to have the DOF I want.



But implicit in “given composition” are several assumptions that are often overlooked:
Same camera position
Same framing

Is there another definition of "composition", other than "perspective and framing"?

[Jac@stafford.net]

his is an opinion I don't share.

Actually it is not an opinion. It is science.


It's logical that the same composition and aperture diameter gives less exposure when it's spread over an 8x10 piece of film vs. half-frame 35mm.

It is a good thing we use f-stops rather than absolute aperture diameter for exposure calculations, isn't it?

[Leigh]

This is an opinion I don't share. It's logical that the same composition and aperture diameter gives less exposure when it's spread over an 8x10 piece of film vs. half-frame 35mm. Fundamentally, I have no reason to expect a given aperture to result in the same exposure time for vastly different camera formats, but gee, wouldn't it be nice if the same aperture number resulted it the same, know, aperture, so that I could count on the image physics being the same.
It's obvious that you don't understand the meaning of the "f" in "f/stop".

It stands for the lens focal length.

The aperture diameter at f/4 on a 100mm lens is 25mm*, while
the aperture diameter at f/4 on a 300mm lens is 75mm.

Which is exactly what you say you want.

- Leigh

*Note: To be more precise f/n yields the diameter of the entrance pupil, which may differ from the measured diameter of the physical opening in the aperture diaphragm.

[Vaughn]

Thanks, folks! I have learned things I probably should have already known -- or else have forgotten over the that 35 years because I have had no practical use for the information in my particular way of photographing (primarily landscapes).

What is nice about viewing the GG is that one can see the changes in the DoF as one closes down the lens. If the foreground comes into focus on the GG before the background does as I close the lens down, then I know I need to focus a little farther out into the scene -- and visa versa. When I get the focus of both near and far happening at the same time as I close the lens down, then I know I have placed the plane of focus properly, including and tilts/swings. Then I go around and see what f/stop I have, and generally close the lens down another stop or two (usually two) just to compensate for errors with my eye-sight.

The 'science' behind it all is good to know, as long as it does not interfer with the creative flow of creating an image, and with some folks it helps with that creative process. I have a beautiful partial Protarlinse VII casket set for the Speed Graphic and the Compur shutter is marked in mm, and a nice little chart listing all the focal length combinations and the resulting f/stop value at any aperture opening. Never have used it...if I shoot tiny little 4x5's it is for ease and transportbility; and dealing with a casket set in the wilds just seems awkward -- especially figuring out the chart as the light is changing. YMMD, of course. Some folks love them and make wonderful images with them.

[cowanw]

It's obvious that you don't understand the meaning of the "f" in "f/stop".

It stands for the lens focal length.


or not.

•In 1895 John A. Hodges first champions the 'fractional number' system (which he abbreviated to 'F-number') in defiance of the Photographic Society of Great Britan's use of the 'Uniform System (U.S.)' This is the first recorded instance of the 'fractional number' and is likely the original meaning of the 'f' in F-stop.
•In 1901 C. Welborne Piper first proposes a unified system of describing aperture marking called the 'f-diamater' (or fractional diameter) after observing similarities between a half-dozen of the more popular methods of the day.
•From the early 1900's through about 1920, the most common way to refer to Piper's unifying system was as the 'f-number.'
•In 1961 the American national Standards Institute (ASA) officially adopted the 'f-number' as the specification for photoelectric photographic meters. Essentially codifying the term and making 'f-number' the common phrase used to describe aperture for camera manufacturers of the day.
•While the phrase 'f-number' has morphed somewhat since 1961 to the now more commonly used 'f-stop' we use today, there is currently no standardized and generally accepted definition for the 'f' in f-stop. In fact, it has variously been referred to as focal, factorial, fractional, and many other designations by a wide variety of sources over the last 120ish years to the point where even though the system itself has been standardized, there simply is no singular recognized designation for the 'f' in f-stop.

[Patrick13]

The f/stop point is moot because the final DoF you're interested in is the circle of confusion and that is dependent on the size of the final print.

Source:
Since the final-image size is not usually known at the time of taking a photograph, it is common to assume a standard size such as 25 cm width, along with a conventional final-image CoC of 0.2 mm, which is 1/1250 of the image width. Conventions in terms of the diagonal measure are also commonly used. The DoF computed using these conventions will need to be adjusted if the original image is cropped before enlarging to the final image size, or if the size and viewing assumptions are altered.

PS: I was going to be mean, but decided to be helpful instead. There's nothing stopping a person from inscribing their own shutter face plate with the diameters wanted instead of fractional stop values, get on that!

[BetterSense]

It's obvious that you don't understand the meaning of the "f" in "f/stop".

Of course I do. I am not new to optics. Optics is actually my job. I print entire transistors in less space than the film CoC of 35mm. My experience with non-photography optical systems is what leads me to find f-stops an annoying unit. The rest of the optics world uses aperture diameter D and numeric aperture NA. The f-stop of the lens that happens to be nearest the subject is...nothing in particular. It is only used in photography because some guy a long time ago thought it would make exposure determination easier across camera formats. It does, but at the direct trade-off cost of making DOF determination difficult across camera formats in exactly the same way. It is, in fact, a matter of opinion which unit is better.

If lenses were marked with D, then photographers would be able to internalize DOF in terms of composition and D, and it would be valid for any format from minox to ULF. And we would have students coming here asking "why does the same aperture (D) give different exposure on different film sizes". And the answer would be easy and intuitive: if your film is twice as big, it needs twice as many photons, so either expose twice as long or use one stop larger D.

When trying to equalize brightness at the film, make the f-stop the same. When trying to equalize DOF, make the D the same.

I am a photographer. I use lenses to capture light. Apart from composition, the only thing more fundamental to image formation is D. Both together determine system NA which, aside from the definition of blurriness, wavelength, etc. completely determines DOF. D is a fundamental image principle. Exposure is a practical concern arising from imperfect capture texhnology. Units that intuitively expose the principles of image formation are superior to those that prioritize practical concerns while obfuscating the fundamentals. I believe that if photography had had auto-exposure from the start, we would measure aperture in D, we would have a very similar scale of numbers (1.4, 2, 2.8, 4...) for aperture only they would be millimeters, and we would have "sunny 16"-type rules for DOF, valid for any camera format.

[Mark Woods]

Should I introduce the "T/Stop" to the discussion? ;-)

[Dan Fromm]

Should I introduce the "T/Stop" to the discussion? ;-)

No. Its well known that all lenses used in large format photography have 100% transmission.

[Mark Woods]

Its well known that all lenses used in large format photography have 100% transmission.

Really? Air to lens surfaces, compound elements, surface coatings (internal and external) don't increase/decrease the light transmission? This is good news to hear!!

[Jac@stafford.net]

trade-off cost of making DOF determination difficult across camera formats in exactly the same way.

I wonder why my less-than-brilliant mind has no trouble whatsoever with finding the appropriate DOF regardless of format. Could it be the decades of brain-washing and that evil thing called Science? Maths?

[Dan Fromm]

I'm sorry, Mark, [sarcasm] [\sarcasm] didn't insert as I wanted them to. The forum's software may need an upgrade.

More seriously, t/stops have nothing to do with DoF and you know it.

[Ian Greenhalgh]

Excellent thread, one of the most informative I have read in a long time. I hope it doesn't get derailed by sarcastic, argumentative and unpleasant people.

[Tin Can]

+1


Of course I do. I am not new to optics. Optics is actually my job. I print entire transistors in less space than the film CoC of 35mm. My experience with non-photography optical systems is what leads me to find f-stops an annoying unit. The rest of the optics world uses aperture diameter D and numeric aperture NA. The f-stop of the lens that happens to be nearest the subject is...nothing in particular. It is only used in photography because some guy a long time ago thought it would make exposure determination easier across camera formats. It does, but at the direct trade-off cost of making DOF determination difficult across camera formats in exactly the same way. It is, in fact, a matter of opinion which unit is better.

If lenses were marked with D, then photographers would be able to internalize DOF in terms of composition and D, and it would be valid for any format from minox to ULF. And we would have students coming here asking "why does the same aperture (D) give different exposure on different film sizes". And the answer would be easy and intuitive: if your film is twice as big, it needs twice as many photons, so either expose twice as long or use one stop larger D.

When trying to equalize brightness at the film, make the f-stop the same. When trying to equalize DOF, make the D the same.

I am a photographer. I use lenses to capture light. Apart from composition, the only thing more fundamental to image formation is D. Both together determine system NA which, aside from the definition of blurriness, wavelength, etc. completely determines DOF. D is a fundamental image principle. Exposure is a practical concern arising from imperfect capture texhnology. Units that intuitively expose the principles of image formation are superior to those that prioritize practical concerns while obfuscating the fundamentals. I believe that if photography had had auto-exposure from the start, we would measure aperture in D, we would have a very similar scale of numbers (1.4, 2, 2.8, 4...) for aperture only they would be millimeters, and we would have "sunny 16"-type rules for DOF, valid for any camera format.

[Vaughn]

...[f-stops] It is only used in photography because some guy a long time ago thought it would make exposure determination easier across camera formats. It does, but at the direct trade-off cost of making DOF determination difficult across camera formats in exactly the same way. It is, in fact, a matter of opinion which unit is better.

If lenses were marked with D, then photographers would be able to internalize DOF in terms of composition and D...

Perfectly valid and well stated, but it begs the questions, how many photographers want to, or need to, make precise DoF determinations? How many photographers prefer to make exposure determinations as easy as possible? I have a process lens marked in both f-stops and mm, so must have been useful in the field.

[Jeff Conrad]

...have you ever seen a camera with auto-DOF?

Almost. In years past, Canon’s high-end bodies (e.g., EOS-5, EOS-3, EOS-1v, and EOS-1D/Ds had a “depth-of-field AE” mode in which the user would focus on the far, focus on the near, and depress the focus button again to set the focus and aperture. The process was fast and deterministic, much the same as doing this from the focus spread on a view camera or with distance and DoF scales on manual-focus hand-camera lenses. For reasons known only to themselves, Canon dropped this feature when the EOS-1D Mk II was introduced in 2004. Some of the lower-end cameras had (and may still have) a feature known as “ADEP,” in which the camera chooses the near and far points. For some things—perhaps portraits—this may work fine, but for others—such as landscape or architecture—the near and far points are usually too close together. And in any event, the choice of DoF limits is beyond the user’s control.

So in many situations, it’s harder to quickly determine focus and f-number with a $7000 digital wonder than it was with a $150 Pentax K-1000—or with almost any view camera. Progress ...


Maybe you don't switch formats often.
I sometimes do, but almost never take the same composition in two different formats. Obviously some people do—as I said, YMMV.


Is there another definition of "composition", other than "perspective and framing"?
Certainly not for me, but for some reason, almost every discussion like this has folks who talk of using the same lens for different formats and adjusting the framing by changing the camera position.

[Jeff Conrad]

•In 1961 the American national Standards Institute (ASA) officially adopted the 'f-number' as the specification for photoelectric photographic meters. Essentially codifying the term and making 'f-number' the common phrase used to describe aperture for camera manufacturers of the day.

The term was used at least as early as 1948, in ASA Z38.2.6-1948, American Standard for General-Purpose Photographic Exposure Meters (Photoelectric Type), which refers to “relative aperture or f-number.” This may not be the first such use; the standard refers to an earlier one—Z38.4.7-1943—of which I don’t have a copy.


... there is currently no standardized and generally accepted definition for the 'f' in f-stop. In fact, it has variously been referred to as focal, factorial, fractional, and many other designations by a wide variety of sources over the last 120ish years to the point where even though the system itself has been standardized, there simply is no singular recognized designation for the 'f' in f-stop.

For sure. And the same is true for giving the value, which may appear as f/16, f/16, f-16, f16, or perhaps otherwise. The most logical interpretation to me is the first, in which f is set in italics (strictly, in oblique font) because it’s a quantity symbol, the lens focal length. Another interpretation is that it’s in italics because it’s a letter used as a word. In The Manual of Photography (9th ed., 2000), Sidney Ray suggests that the indication f/ “serves as a reminder of its derivation.” We usually pronounce it as “eff sixteen” rather than “eff over sixteen,” so who knows? Some years ago, I got sucked into a protracted and insane discussion about this until I concluded that I had better uses for my time.

[Mark Woods]

Hello Dan, I was hoping for the sarcasm light to go on on my computer. Failure at this end. But T/Stops [I]are[I] based on light transmission while F/Stops are mathematically determined. If one tested one's lens with light transmission, one would most likely see that the T/Stop is always more open than the F/Stop, and the F/Stop is what is used for DOF. I keep this in mind when I'm shooting to give about 1/3 stop more exposure on my older lenses. I have one lens that is 1 full stop slower (i.e., to get the right exposure I need to open the lens by one full stop either time or F/Stop). The light transmission was mentioned earlier, and that system does exist.

Happy Holidays!

[Dan Fromm]

Hello Dan, I was hoping for the sarcasm light to go on on my computer. Failure at this end. But T/Stops [I]are[I] based on light transmission while F/Stops are mathematically determined. If one tested one's lens with light transmission, one would most likely see that the T/Stop is always more open than the F/Stop, and the F/Stop is what is used for DOF. I keep this in mind when I'm shooting to give about 1/3 stop more exposure on my older lenses. I have one lens that is 1 full stop slower (i.e., to get the right exposure I need to open the lens by one full stop either time or F/Stop). The light transmission was mentioned earlier, and that system does exist.

Happy Holidays!
Thanks, Mark, for reassurance that it wasn't my fault. It usually is.

Yes, I know that t/ stops are photometric and that f/ stops are geometric. I was once badly caught out by the difference. I had a 4008ZM with an Angenieux 8x8B, an f/1.9er. While I was shooting a travel film in Costa Rica the the camera's exposure system went berserk. I hadn't tested (shame on me) and the few shots I took with the Beaulieu using an external meter were badly underexposed. Fortunately I had a back-up camera, switched to it because I knew the 8x8B was dim dim dim. When I got home I tested. f/1.9 lens, so engraved, t/3.3, so measured. Ouch.

Season's greetings,

Dan

[Dan Fromm]

I am a photographer. I use lenses to capture light. Apart from composition, the only thing more fundamental to image formation is D. Both together determine system NA which, aside from the definition of blurriness, wavelength, etc. completely determines DOF. D is a fundamental image principle.

"Both together"? How?

Ignorant barbarian that I am, I've always understood that N.A. = n sin θ, where n is the medium's index of refraction (air's is 1.0) and θ is half the angle of the maximum cone of light that can enter the lens. Also that the relationship between the f/ number N and N.A. is N = 1 /(2 N.A.). Ain't no dimensions in either relationship, so where does your D fit? Are you sure you know what you're talking about?

[Mark Woods]

Thanks Dan. You and I know the difference, but I'm sure there are a few who don't know.

Enjoy the holidays. I always enjoy your posts.

Mark

[BetterSense]

Almost. In years past, Canon’s high-end bodies (e.g., EOS-5, EOS-3, EOS-1v, and EOS-1D/Ds had a “depth-of-field AE” mode in which the user would focus on the far, focus on the near, and depress the focus button again to set the focus and aperture.

Of all the automatic camera features, the one I want the most, and have never seen, is a hyperfocal mode, where focus is automatically set to the hyperfocal distance for whatever aperture is chosen, whether by the photographer or the auto-exposure system. Never seen it...


"Both together"? How?

Thin lens formula usually given for DOF: DOF=(lambda*n)/NA^2 + n*theta/(M*NA). The only two terms, other than wavelength, are NA and M (e is like CoC). For a given composition and D, you have determined M and NA, thus DOF.

Note that M here is final print magnification. It should be obvious that the final magnification and not magnification at some intermediate, internal plane (like a film surface)

[djdister]

All I can say is - TALK IS CHEAP. How come all you smart fellers haven't posted any photographic examples which illustrate your points? This is a PHOTOGRAPHY forum, after all...

[Mark Woods]

Hello Dan, I believe that to present photos representing the above points would require A) a scanner that resolves 1:1 the print resolution, B) a monitor that resolves the print resolution 1:1, & C) a defined distance to view the print. I'm afraid A&B don't really exist, but we could say, e.g., "Hold your monitor 2' away for evaluation purposes."

A large part of photography is the science and the discussion of the science, techniques, and ultimately does it lead to artistry. The first two can be scientifically evaluated, while the third is strictly an opinion.

Enjoy the Holidays!

Mark

[Dan Fromm]

All I can say is - TALK IS CHEAP. How come all you smart fellers haven't posted any photographic examples which illustrate your points? This is a PHOTOGRAPHY forum, after all...

I hardly do digital and the one scanner I own, a gift Nikon 4000 ED, has failed POST ever since I first turned it on. Some years ago, when Kodachrome could be processed, I did some test shots whose point was that most of the posts in this discussion are utter nonsense. Small format, still germane.

Three shots, all at 1:4, all on KM, all of a decoration, a sort of potted plant made of beads on wires, all with the same illumination, all at f/8. 55/2.8 MicroNikkor AIS at f/8, 105/2.8 MicroNikkor AIS, 700/8 Questar (t/11, with flash power adjusted to keep exposure the same as in the other two shots). I chose 1:4 because that's the Questar's magnification at its close focusing limit. The resulting transparencies were indistinguishable. Equally sharp, same DoF. The Questar 700 really is a sharp lens, but hard to get really good results from in the field because that demands absolute steadiness. If you don't believe my results, try a similar experiment yourself.

If you do the calculations, you'll find that there is one situation in which a short lens will give more DoF than a long lens at the same magnification and aperture (f/ number) and with the same circle of confusion. Same magnification is equivalent to same front node-to-subject distance, measured in focal lengths. This is when the magnification at which the shot is taken puts the subject at or near the short lens' hyperfocal distance and far closer to the long lens than its hyperfocal distance. For shots where the subject is at or beyond both lenses' hyperfocal distances, they'll give the same DoF.

If you don't believe these results, play with my spreadsheet. You can download it from http://sdrv.ms/19o1T1C If you check the formulas and find an error, please tell me about so that I can correct it.

[Jeff Conrad]

If you do the calculations, you'll find that there is one situation in which a short lens will give more DoF than a long lens at the same magnification and aperture (f/ number) and with the same circle of confusion.
Indeed. For an analytical treatment, see my paper at http://www.largeformatphotography.info/articles/DoFinDepth.pdf. Eq. (24) predicts Dan’s calculations. When the distance is small in comparison with the hyperfocal distance, this simplifies to Eq. (25), which is what is assumed in most discussions. For much general photography, this is reasonable, but it’s important to recognize that it’s an approximation valid only for certain conditions. Dan’s calculations also clearly show that the venerable “1/3:2/3” front:rear DoF distribution is valid only at one distance (about 1/3 the hyperfocal distance). Though this is hardly news, the myth just refuses to die ...

Of course, constant magnification isn’t quite the “same composition,” so it’s important to state just what is being compared before making sweeping generalizations. I address DoF vs. camera format for “same composition” in, of all places, the section Depth of Field and Camera Format. The common assumptions are given in Eq. (76) (f-number is proportional to format size for constant DoF) and Eq. (77) (DoF is inversely proportional to format size for constant f-number). These assumptions are good for rough comparisons for many situations in general photography, but it’s important to realize that they’re approximations that are valid only over a limited range of distances.

So which approach is right? As before, it all depends on your objective ...

P.S. Dan, it might be helpful to add columns showing u/H for both sheets, and perhaps showing magnification for the “for a lens” sheet. It might also help to have a cell showing magnification for the sheet “constant magnification.”

It’s also worth noting that the “max res” is approached only in the plane of focus—the resolution at DoF limits will always be less. But as long as the resolution is sufficient for the unsharpness to be essentially undetectable, it’s good enough—after all, that’s what DoF is about.

[Leigh]

The f/stop point is moot because the final DoF you're interested in is the circle of confusion and that is dependent on the size of the final print.
Circle of Confusion is absolutely irrelevant to DoF ON THE NEGATIVE.

It's a common fudge factor based on assumptions about the size of the print being made.
It's as valid as any other assumption.

- Leigh

[Vaughn]

The most important factor is there to be no confusion about why one takes an photograph. If any of this information helps one get there, then great! Merry Christmas all!

Waiting for my boys to finish up the last preparations for Christmas dinner! Catalina Cranberry Chicken, cranberry Jell-O salad, rice, green salad, and then cranberry-apple pie for desert! All I had to do was cook the rice (and buy the food!)

Just got the "It's ready" call!!!

[Dan Fromm]

Indeed. For an analytical treatment, see my paper at http://www.largeformatphotography.info/articles/DoFinDepth.pdf. Eq. (24) predicts Dan’s calculations. When the distance is small in comparison with the hyperfocal distance, this simplifies to Eq. (25), which is what is assumed in most discussions. For much general photography, this is reasonable, but it’s important to recognize that it’s an approximation valid only for certain conditions. Dan’s calculations also clearly show that the venerable “1/3:2/3” front:rear DoF distribution is valid only at one distance (about 1/3 the hyperfocal distance). Though this is hardly news, the myth just refuses to die ...

Of course, constant magnification isn’t quite the “same composition,” so it’s important to state just what is being compared before making sweeping generalizations. I address DoF vs. camera format for “same composition” in, of all places, the section Depth of Field and Camera Format. The common assumptions are given in Eq. (76) (f-number is proportional to format size for constant DoF) and Eq. (77) (DoF is inversely proportional to format size for constant f-number). These assumptions are good for rough comparisons for many situations in general photography, but it’s important to realize that they’re approximations that are valid only over a limited range of distances.

So which approach is right? As before, it all depends on your objective ...

P.S. Dan, it might be helpful to add columns showing u/H for both sheets, and perhaps showing magnification for the “for a lens” sheet. It might also help to have a cell showing magnification for the sheet “constant magnification.”

It’s also worth noting that the “max res” is approached only in the plane of focus—the resolution at DoF limits will always be less. But as long as the resolution is sufficient for the unsharpness to be essentially undetectable, it’s good enough—after all, that’s what DoF is about.

Jeff, I'm ashamed to admit that if I ever was aware of your paper, awareness of it has long since fallen out of my mind. Thanks very much for posting a link to it.

Thanks also for your suggestions re my little spreadsheet. I didn't actually write it as a DoF calculator, I did it to clarify a point that Helen Bach and I had been discussing without converging on agreement. This in '06. I wasn't clear enough and, unusually for her, she missed the signal in the noise. That's why I took the easy way out and made focused distance (in focal lengths) a variable instead of making magnification a variable and calculating focused distance from that. They're equivalent, and focused distance (in focal lengths) was enough for Helen and me to understand where we agreed and why we didn't agree on everything.

I worry about DoF primarily when shooting closeup, where loss of resolution on stopping down can easily turn DoF calculations into nonsense. I have another little spreadsheet that I've used to design macro flash rigs (for magnifications 1:20 to 2:1, 2:1 is often above the practical limit in the field); it shows what the flash design will do and, more as interesting information than as a guide to how well the flash design will work, DoF and the diffraction limit in the plane of best focus. At higher magnifications 1/(diffraction limit) is sometimes larger than the CoC used in calculating DoF. This may be one of those things that isn't meant to be known.

Leigh, there's something about discussions of DoF that makes common sense evaporate. I don't know what it is, but you're right, this discussion has lost sight of what matters, viz., the final print.

[Tin Can]

Jeff Conrad, your paper is a very thorough treatise. I had to blur over a lot of the math, but fortunately for me you inserted very understandable plain english. I got a lot out of your paper.

Thank you!

[Bill Burk]

I'd taken a pair of negatives of the same composition, one on 35mm film with a 40mm lens (I think at f/8), and one on 4x5 with a 127mm lens (I think at f/16).

Never got around to making prints and analyzing the difference/similarity. I think they looked similar, except that I could read the house number across the street from the 4x5 negative.

[Jeff Conrad]

[your spreadsheet] That's why I took the easy way out and made focused distance (in focal lengths) a variable instead of making magnification a variable and calculating focused distance from that.
There are many ways to look at most things, and each approach can be informative. I’m convinced there is some simple parameter that will reveal all of practical value about DoF ... but I’ve yet to determine just what that parameter is. My objective in analysis is to come up with a few general principles that make it easier to get the pictures I want, and perhaps even more to identify the stuff about which I need not concern myself—which seems to be most stuff that people spend untold hours discussing.


I worry about DoF primarily when shooting closeup, where loss of resolution on stopping down can easily turn DoF calculations into nonsense.
Yep. Diffraction is vastly overrated in most practical photography, but it can be a big deal in closeups. Though I probably don’t stress it enough, the curves that I show in the section Optimum f-Number from MTF are for effective f-number (i.e., marked f-number at infinity and zero magnification). Curves for marked f-number at magnifications approaching unity are strikingly different; it’s really easy to stop down so far that sharpness decreases noticeably, even at the DoF limits.


... there's something about discussions of DoF that makes common sense evaporate.
No foolin’ ... unfortunately, the beatings will likely continue until morale improves ...

[Dan Fromm]

it’s really easy to stop down so far that sharpness decreases noticeably, even at the DoF limits.

Take a look at H. Lou Gibson's little pamphlet Photomacrography (Kodak Publication N12-B, incorporated in Kodak Publication N-16 Closeup Photography and Photomacrography). He does the calculations and shows example photographs in which the zone of acceptable sharpness shrinks on stopping down. There are limits to what can be done.



perhaps even more to identify the stuff about which I need not concern myself—which seems to be most stuff that people spend untold hours discussing.

Focus is a real problem.

[mihag]

Indeed. For an analytical treatment, see my paper at http://www.largeformatphotography.info/articles/DoFinDepth.pdf. Eq. (24) predicts Dan’s calculations.

Thank you, I'm already looking forward to reading it.

Another potentially intersting article on DoF (and bokeh) from Carl Zeiss: http://tinyurl.com/265vg8p

[Jim Jones]

Of all the automatic camera features, the one I want the most, and have never seen, is a hyperfocal mode, where focus is automatically set to the hyperfocal distance for whatever aperture is chosen, whether by the photographer or the auto-exposure system. Never seen it...

Depth of field scales, which give hyperfocal distances, were common on the better small and medium format film cameras. In the absence of such scales, the hyperfocal distance for critical work with a normal focal length lens is about 2000 times the aperture as seen through the front elements of the lens if the final image is viewed in correct perspective. For lenses with shorter or longer focal lengths, the hyperfocal distance appears to be less or greater, proportional to that lens' focal length in relation to a normal lens. This is because the viewing distance is often consistent, even though the focal length varies. A DOF scale can be improvised and added to the rail or bed of many view cameras. This gets a little complicated when a variety of lenses or print sizes are involved. I'll leave it to someone with spreadsheet experience to do the calculations. I did it in the long-gone days of pencil, paper, and slide rule, which gave me a practical understanding of DOF and hyperfocal distance. Once it is understood, the math becomes largely irrelevant.

[Jac@stafford.net]

Of all the automatic camera features, the one I want the most, and have never seen, is a hyperfocal mode, where focus is automatically set to the hyperfocal distance for whatever aperture is chosen, whether by the photographer or the auto-exposure system. Never seen it...

While not the same, earlier Hasselblad lenses have shutter duration coupled to f-stops - a feature I dislike but live with.

[djdister]

Thank you, I'm already looking forward to reading it.

Another potentially intersting article on DoF (and bokeh) from Carl Zeiss: http://tinyurl.com/265vg8p

The Zeiss article was excellent, and exactly what I was suggesting is needed to augment this line of discussion! It has just enough math, plenty of graphs, illustrations of optical principles and even photographic examples for comparison. Thanks for posting a link to it.

[Nathan Potter]

Dan, I had forgotten about that Kodak publication N12-B and just found it in a storage box. Mine is just N12 and includes A and B. It is old, 1977, and I suspect may come up occasionally on Ebay. Worth getting, but the closeup technology has changed greatly since then, where now focus scanning is state of the art for DSLR work.

Returning to the OP queries I can say what I do about DOF estimation when I want to set a far limit of resolution at infinity and want to know where the near limit is.
Using standard formula for determining the near limit and far limit I just generate a pocket chart for each lens I use which indicates the hyperfocal distance, the near limit, far limit set at ∞ and for a COC of 20µm and 40 µm. Then, often without shifts or tilts, I'll estimate the hyperfocal distance and get that in focus on the GG and go from there. Recently I use a rangefinder; when I remember it. Deriving my charts assume no magnification factors. The near and far limits are computed by simple algebra thusly:

Df =µf^2/(f^2-NCµ) for the far limit and Dn-µf^2/(f^2+NCµ) for the near limit.

Df = far limit
Dn = near limit
µ = object conjugate distance
f =lens focal length
N = f/no.
C = circle of confusion

If I ignore a near and far limit and just want a depth of field then DOF = 2µ^2NC/f^2.

To determine the hyperfocal distance which assumes the far limit at ∞ I use: H = f^2/NC + f where H = hyperfocal distance.

So a recently done pocket chart for the Schneider 47 mm XL is shown below.

f/no. H in ft. RANGE (ft) H in ft. RANGE (ft)
(COC = 20 µm) (COC = 40 µm)
5.6 165 82 to ∞ 82 41 to ∞
8 45 22 to ∞ 22 11 to ∞
11 33 16 to ∞ 16 8 to ∞
16 22 11 to ∞ 11 5 to ∞
22 16 8 to ∞ 8 4 to ∞
32 11 5 to ∞ 5 3 to ∞
45 8 4 to ∞ 4 2 to ∞
64 5.5 2.7 to ∞ 2.7 1.4 to ∞

A really handy field chart for a couple of common Circles of Confusions for critical work with the caveat of finding the hyperfocal distance with some degree of accuracy. Note that the 47 XL approaches a pinhole for DOF but the optics provides much better resolution when needed.

Nate Potter, Austin TX.

[Nathan Potter]

Well crap and sorry, the Forum software doesn't preserve the spacing for charts; but maybe one can figure it out. If there is some interest I can post some actual charts.

Nate Potter, Austin TX.

[Dan Fromm]

Dan, I had forgotten about that Kodak publication N12-B and just found it in a storage box. Mine is just N12 and includes A and B. It is old, 1977, and I suspect may come up occasionally on Ebay. Worth getting, but the closeup technology has changed greatly since then, where now focus scanning is state of the art for DSLR work.

Yes it has, hasn't it? The ideas are still good, though, and transfer from small format (Instamatics!) for which the Closeup pamphlet was written, to larger formats.

Unfortunately the scanning approach fails completely for the subjects that got me into closeup photography. Unconstrained fish in aquaria. Kinda like SEM work. Infinite DoF and high resolution but the subject has to be safely dead.

Funny thing is, the only time I worry about DoF when I'm shooting out and about is when I want to isolate the subject from the background. This involves shooting at a relatively large aperture at which viewing on the GG is easy. Interestingly, with some of my faster lenses I can see the trade-off between isolating the subject and pleasing rendition of the background. I sometimes have to give up a little of one to get more of the other. I'm not sure that calculations would help much there.

Cheers,

Dan

[Nathan Potter]

Motion is not friendly to closeup work. That would be pure frustration for me.

By the way, in my post above I picked 20 and 40 µm as a COC and forgot to mention that a diffraction limit cuts that to a larger value at the point in f/stop where the COC approximately equals the diffraction limit.

So, for example, with the 47 XL at f/22 the COC is about 20 µm; at f/32 the COC is about 30µm and at f/45 the COC is about 40 um and so on. This from the diameter of an Airy disk which is D = 2.44 X lamda X N, or roughly 1.22 N at .5 µm.

In fact about the only time I worry about DOF is for landscape work where I want the near and far in fine focus.

Nate Potter, Austin TX.

[Leonard Evens]

Jeff Conrad's discussion does a good job of explaining what is going on, and anyone who really want to understand the subject should read his article.

But I'm firmly of the belief that there is no way to really understand the subject without going through the mathematics, amply described in Jeff's article.

I notice that no one made any effort to go into the mathematics, and, perhaps, that is the wisest course, but I will take a chance and do so.

DOF is a mathematical concept in geometric optics, and if you are willing to make the effort, a lot that seems mysterious becomes clear. Let me illustrate my point with the formula for hyperfocal distance.

H = f + f^2/(Nc)

where f is the focal length of the lens, N is the f-number used, and c is the diameter of the largest acceptable circle of confusion. (Since usually f is much smaller than f^2/(Nc), one can usually get by with the approximation

H ~ f^2/(Nc)
)

If you know the hyperfocal distance, you can calculate everything you may want to know about depth of field.

The quantity c is what is missing from most of the previous responses. It is a measure of how sharp you need the image to be in the negative in the camera. Unless one is making contact prints, one is more interested in what the viewer of the final print sees, and the final print will be enlarged from the image in the camera. Thus, typically, to make an 8 x 10 print, a 35 mm negative must be enlarged about eight times, while a 4 x 5 negative must be enlarged twice. That means you need to use a value of c about one fourth as large for a 35 mm negative as for a 4 x 5 negative. That in turn means that if the focal length f and f-number N are the same, you will get a hyperfocal distance about four times larger for the same size final print with a 35 mm format image than with a 4 x 5 format image.

When you focus at the hyperfocal distance, everything (in the final print) between half the hyperfocal distance and infinity will be in focus. So, in that case, the larger the hyperfocal distance the less the depth of field, which means that for the same focal length lens and the same f-number, you will have significantly less depth of field (in the final print) with 35 mm format than with 4 x 5 format.

If you look at the formulas which apply when focused at any other specific distance, you see that the same is true. With the same focal length and the same relative aperture, you have significantly less depth of field if the original format is 35 mm than if it is 4 x 5.

But you have to be careful about generalizing this. Similar calculations show that if you change the focal length so that the angle of view is the same in the two cases, e.g., so the 35 mm format focal length is about one fourth of the 4 x 5 format focal length, then the depth of field (in the final print) is significantly greater if the original image is in 35 mm format than if the original image is in 4 x 5 format.

This is no a contradiction, if you keep in mind that while, for example, 100 mm is a relatively long lens for 35 mm format, it is relatively short lens for 4 x 5 format. Of course if the format stays the same, longer lenses have less depth of field than shorter lenses, but to see how this plays out when you change formats, you need to look at the mathematics.

(In particular, if in the formula f^2/(Nc) you multiply c by 4 but keep f the same, you divide the total fraction by 4. But if you multiply c by a factor of 4 and at the same time multiply f by the same factor, because f^2 appears in the numerator, the total effect is to multiply f^2/(Nc) by 4. In other words, just multiplying c by 4 has the opposite effect to multiplying c by 4 and at the same time multiplying f by 4.)

Note that there is no way to understand how these different factors play out in specific situations without doing the mathematics. A purely verbal description can't do it.

[Leigh]

earlier Hasselblad lenses have shutter duration coupled to f-stops - a feature I dislike but live with.
Modern (Prontor shutter) Hasselblad lenses still have the shutter/aperture coupling.

The difference is with the Compur shutters, it was engaged by default and you had to press a lever to decouple the two.

With the Prontor shutters, that feature is disengaged by default, and you must press a lever to couple them.

- Leigh

[Tin Can]

Perhaps a few images could help. Some of us, like me, need all the help visually we can find. Of course there are infinite variation, but portrait, interior and something distant could illustrate this in 6 images. Use extremes for examples.

Thanks for everything so far!

btw, I have forgotten how to solve H ~ f^2/(Nc) that little '^' eludes my memory!



Jeff Conrad's discussion does a good job of explaining what is going on, and anyone who really want to understand the subject should read his article.

But I'm firmly of the belief that there is no way to really understand the subject without going through the mathematics, amply described in Jeff's article.

I notice that no one made any effort to go into the mathematics, and, perhaps, that is the wisest course, but I will take a chance and do so.

DOF is a mathematical concept in geometric optics, and if you are willing to make the effort, a lot that seems mysterious becomes clear. Let me illustrate my point with the formula for hyperfocal distance.

H = f + f^2/(Nc)

where f is the focal length of the lens, N is the f-number used, and c is the diameter of the largest acceptable circle of confusion. (Since usually f is much smaller than f^2/(Nc), one can usually get by with the approximation

H ~ f^2/(Nc)
)

If you know the hyperfocal distance, you can calculate everything you may want to know about depth of field.

The quantity c is what is missing from most of the previous responses. It is a measure of how sharp you need the image to be in the negative in the camera. Unless one is making contact prints, one is more interested in what the viewer of the final print sees, and the final print will be enlarged from the image in the camera. Thus, typically, to make an 8 x 10 print, a 35 mm negative must be enlarged about eight times, while a 4 x 5 negative must be enlarged twice. That means you need to use a value of c about one fourth as large for a 35 mm negative as for a 4 x 5 negative. That in turn means that if the focal length f and f-number N are the same, you will get a hyperfocal distance about four times larger for the same size final print with a 35 mm format image than with a 4 x 5 format image.

When you focus at the hyperfocal distance, everything (in the final print) between half the hyperfocal distance and infinity will be in focus. So, in that case, the larger the hyperfocal distance the less the depth of field, which means that for the same focal length lens and the same f-number, you will have significantly less depth of field (in the final print) with 35 mm format than with 4 x 5 format.

If you look at the formulas which apply when focused at any other specific distance, you see that the same is true. With the same focal length and the same relative aperture, you have significantly less depth of field if the original format is 35 mm than if it is 4 x 5.

But you have to be careful about generalizing this. Similar calculations show that if you change the focal length so that the angle of view is the same in the two cases, e.g., so the 35 mm format focal length is about one fourth of the 4 x 5 format focal length, then the depth of field (in the final print) is significantly greater if the original image is in 35 mm format than if the original image is in 4 x 5 format.

This is no a contradiction, if you keep in mind that while, for example, 100 mm is a relatively long lens for 35 mm format, it is relatively short lens for 4 x 5 format. Of course if the format stays the same, longer lenses have less depth of field than shorter lenses, but to see how this plays out when you change formats, you need to look at the mathematics.

(In particular, if in the formula f^2/(Nc) you multiply c by 4 but keep f the same, you divide the total fraction by 4. But if you multiply c by a factor of 4 and at the same time multiply f by the same factor, because f^2 appears in the numerator, the total effect is to multiply f^2/(Nc) by 4. In other words, just multiplying c by 4 has the opposite effect to multiplying c by 4 and at the same time multiplying f by 4.)

Note that there is no way to understand how these different factors play out in specific situations without doing the mathematics. A purely verbal description can't do it.

[Jeff Conrad]

Take a look at H. Lou Gibson's little pamphlet Photomacrography (Kodak Publication N12-B, incorporated in Kodak Publication N-16 Closeup Photography and Photomacrography). He does the calculations and shows example photographs in which the zone of acceptable sharpness shrinks on stopping down. There are limits to what can be done.

Paul Hansma did a similar analysis in the March/April 1996 issue of Photo Techniques “View Camera Focusing in Practice.” Unlike Gibson, Hansma assumed infinity focus, so he didn’t deal with the significant degradation that you get in closeup work. Hansma’s article is available on the site under References in Q.T. Luong’s article How to select the f-stop (http://www.largeformatphotography.info/fstop.html). Despite what I say in my paper, the article is four GIFs rather than PDF.

Gibson and Hansma both assumed that combined defocus and diffraction could be modeled by a root-square combination of defocus and diffraction blur spots. Gibson promised a more complete description of the basis for this but I’m not sure the article was ever written. As I understand it, the root-square (or sometimes linear) combination of resolutions is more a rule of thumb than hard science; however, the results—especially Hansma’s—are quite similar to what I got using calculated MTFs. This would seem to suggest that we’re at least in the ballpark, and that as Dan said, there sometimes are limits to what can be done—especially with closeups.

It’s a very interesting article; Gibson attempts to address some additional causes of image degradation that Hansma and I ignore.

[Dan Fromm]

Motion is not friendly to closeup work. That would be pure frustration for me..

It is at times for me too. I shot fish with KM using a 35 mm SLR, proper macro lens, and flash illumination. All I had to do was get the fish to perform front and center and track them by teetering. The flash stopped motion, both subject and camera.

And then I started shooting flowers with the same gear. Much easier than fish, until I went up in format. Wind kills me. Flash stops motion, but the intended plane of best focus can move between the time I've focused and composed and the time the shutter has been closed and cocked, film holder inserted, dark slide pulled, ...

Jeff, in photography ball park is often close enough. But you know that too.

I've done my calculations, when I bother, to see what's clearly impossible and, as I've mentioned, to design closeup flash rigs. That's an interesting problem that people with auto-everything cameras tend to ignore, but it has some significance for those of us who still use auto-nothing cameras. Following up on a hint in a discussion of Spiratone's MacroDapter in, IIRC, MP, I've designed a number of flash rigs that give good exposure with fixed power flashes and a fixed aperture setting (nominal, not effective) over a useful range of magnifications. If you're interested I can give you a link to the spreadsheet. The current realization of the design incorporates a Jones of Hollywood flash bracked that's similar to the MacroDapter but that offers better geometry. The flash design spreadsheet reports, among other things, the largest enlargement possible given the diffraction limit at the effective aperture. Its just one discouragement after another.

Cheers,

Dan

[ic-racer]

Is is correct that LF lenses and 35mm lense of the same focal length and set to the same f/stop would produce images with different DOF's??? If possible can someone give an "Explanation for Dummies" type answer as to what the difference and why?

The circles of confusion will be the same size, but DOF calculation involves other variables, like viewing distance.

[Heroique]

A few field notes about DOF from a practitioner (not a physicist):


• Aperture – double the f-number (for example, f/16 to f/32), and you double DOF. Use half the f-number (say f/22 to f/11), you get half the DOF. Note to self: See note about diffraction below.
• Camera-to-subject distance – double the distance, you increase DOF 4x (a matter of squaring). So, increase distance 4x, and DOF increases 16x. Decrease distance by 1/2, DOF decreases to 1/4.
• Focal length of lens – reduce it by one-half (300mm to 150mm), DOF increases 4x (a matter of inverse-squaring). Double it (75mm to 150mm), and DOF decreases to 1/4.

Notes about "acceptable" sharpness w/in DOF from a practitioner (not a psychiatrist):


• Degree negative is enlarged for a print – a print may appear "acceptably" sharp, but a larger print from the same negative may appear "unsharp" at the same viewing distance. Your friend may disagree and think both are plenty sharp. That's correct too.
• Distance at which print is viewed – a given print that looks "acceptably" sharp at one distance may very well appear "unsharp" at a closer distance. For example, a few feet vs. 12 inches. If your friend disagrees, tell him that's okay.

-----
Plus a practitioner's note about diffraction:
Even strict standards for coc in prints up to around 16x20 (from 4x5 film) will allow one to use, say, f/45 without generating discernable diffraction. And if the print is smaller, the aperture can be narrower. Sure, diffraction is “setting in” on a theoretic level as you click down from f/22 to f/32 to f/45 and narrower, but to a degree that a field photographer can often ignore, so that more critical issues can be addressed – like performing magic tricks to make the wind subside! :D

[Tin Can]

LOL. My Hero!

Great concise info, that I will PRINT out and take with me, until it becomes, wait I forgot what I was writing...

Right, take this with me.

May I copy and print this, to be shared only with my camera?


A few field notes about DOF from a practitioner (not a physicist):


• Aperture – double the f-number (for example, f/16 to f/32), and you double DOF. Use half the f-number (say f/22 to f/11), you get half the DOF. Note to self: See note about diffraction below.
• Camera-to-subject distance – double the distance, you increase DOF 4x (a matter of squaring). So, increase distance 4x, and DOF increases 16x. Decrease distance by 1/2, DOF decreases to 1/4.
• Focal length of lens – reduce it by one-half (300mm to 150mm), DOF increases 4x (a matter of inverse-squaring). Double it (75mm to 150mm), and DOF decreases to 1/4.

Notes about "acceptable" sharpness w/in DOF from a practitioner (not a psychiatrist):


• Degree negative is enlarged for a print – a print may appear "acceptably" sharp, but a larger print from the same negative may appear "unsharp" at the same viewing distance. Your friend may disagree and think both are plenty sharp. That's correct too.
• Distance at which print is viewed – a given print that looks "acceptably" sharp at one distance may very well appear "unsharp" at a closer distance. For example, a few feet vs. 12 inches. If your friend disagrees, tell him that's okay.

-----
Plus a practitioner's note about diffraction:
Even strict standards for coc in prints up to around 16x20 (from 4x5 film) will allow one to use, say, f/45 without generating discernable diffraction. And if the print is smaller, the aperture can be narrower. Sure, diffraction is “setting in” on a theoretic level as you click down from f/22 to f/32 to f/45 and narrower, but to a degree that a field photographer can often ignore, so that more critical issues can be addressed – like performing magic tricks to make the wind subside! :D

[Jeff Conrad]

Take a look at H. Lou Gibson's little pamphlet Photomacrography (Kodak Publication N12-B, incorporated in Kodak Publication N-16 Closeup Photography and Photomacrography).

Well, I’ll stand by my original statement that there’s a significant error in Gibson’s Eq. (6), p. 97 in the bound Pub. N-16. But it’s not quite as I originally stated. On p. 95, he gives the total DoF as

T = uc/(D – c) + uc/(D – c),

where

u = focused distance
c′ = circle of confusion, projected onto the subject plane; it is equal to c/m, where c is the CoC in the image plane (i.e., the one we normally deal with)
D = diameter of lens aperture stop, f/N

He transforms this so that it’s expressed in terms of overall magnification M (= Em, where E is the negative-to-print enlargement and m is the camera magnification):


T = 2Nc′ (M + 1)/M.

It looks to me that this should have been in terms of the camera magnification m, i.e.,


T = 2Nc′ (m + 1)/m.

Because c′ = c/m, this is the same equation commonly used today:


T ≈ 2Nc (m + 1)/m^2.

As it turns out, I made the same initial error in a discussion four years ago, and corrected it shortly thereafter—if I’d read a paragraph further into my comments, I’d have seen this right away ... aghhh.

I must confess that I’ve never checked to see if this carried over into his calculations; it could simply have be a typesetting error in a couple of spots.

Aside from this, it’s a very interesting article; Gibson is conceptually on target, and attempts to address some additional causes of image degradation that Hansma and I ignore.

Incidentally, I made a PDF of Hansma’s article; I can post it here if anyone is interested.