lens perspective question?

[BetterSense]

If you have a photograph with narrow depth of field such that foreground and distant objects are out of focus and the middle area is in focus, changing the print size will not expand or contract the apparent depth of field in the print.

You are wrong. Changing the print size WILL expand or contract the apparent depth of field in the print. Magnification is, in fact, part of how depth-of-field is defined. It's physics and geometry. You cannot simply wish the magnification term out of the thin-lens depth-of-field equation.

Honestly, I'm surprised this is not obvious. Who among us has not printed an apparently-sharp 35mm contact print, only to find out that it looks OOF at 8x10? Why do we use loupes on our ground glass? According to your statement, if it looks sharp at 4x5 it should be sharp at any size.

"A different maximum circle of confusion also applies for each print size and viewing distance combination": Cambridge in colour.

Absolutely! Depth of field has no meaning outside the context of VIEWING. That is why it is defined, from the beginning, using magnification, which is obviously important when it comes to perception of details (else we would have never invented the microscope). When it comes to viewing reflective media, the circle of confusion has to be CHOSEN based on visual perception. It could be .5mm or it could be 4 inches. Once you have chosen a circle of confusion for the print, THEN you can meaningfully talk about depth of field when taking photographs. Circles of confusion at any intermediate stage (such as on the camera film) is basically irrelevant. You could shoot to a small format and enlarge 10x or shoot to a format twice as big and enlarge 5x. The 'film CoC' will be different in each case but the depth of field will be the same.

At at agreed-upon print CoC, all photographs taken at the same aperture (as distinct from f-stop) and enlarged to the same magnification will have the same depth of field characteristics, completely regardless of lens focal length, film format, or whether you are using 8mm or 20x24 film.

[E. von Hoegh]

This has puzzled me all my life. I will look with interest at the concensus. Although I expect we do need to define how close we stand to view the variously sized prints. I suspect the fact that COC is not a constant is the source of consternation.

"A different maximum circle of confusion also applies for each print size and viewing distance combination": Cambridge in colour.

Be sure to apply your tetrapyloctomy filter. (winking smiley)

[Steve Smith]

I think this comment on Luminous Landscape gets it right!

Circle of Confusion (COF)
Definition: "A group of photographers sitting around trying to understand Depth of Field"

Steve.

[E. von Hoegh]

I think this comment on Luminous Landscape gets it right!




Steve.

Perfect!

[RichardSperry]

At at agreed-upon print CoC, all photographs taken at the same aperture (as distinct from f-stop) and enlarged to the same magnification will have the same depth of field characteristics, completely regardless of lens focal length, film format, or whether you are using 8mm or 20x24 film.

This may be true(and what Bob Solomon wrote a out the 210 lens) is true. It's not really useful in a practical sense.

If you take a photo of a person(or subject) that fits in the small format, he/it will be entirely so small on the large format that he/itwill be lost. People don't photograph in such a vacuum, they will move the camera or the subject to fill the frame(they usually change the lens though).

And when that happens, that changes the whole picture.

In Bob's experiment, the only believably realistic people who would photograph this way are going to be people experimenting with depth of field, depth of focus, and perspective.

And that is only useful for people, the kind of people who take photos of test patterns, to prove that a lens does what it does. Which leaves the rest of us changing where the lens(or focal length of it) is to compose our photos normally, which will change the amount of out of focus regions of our photos.

The background blur of a 210 lens taking a head and shoulders photo of a person is going to look different when taken with a 35mm camera than it will with a 4x5 camera. Because the distance from the lens to the subject will change, if that doesn't change it's a completely different photo isn't it. One will be a head and shoulders portrait the other will be head to toe with a bunch of extra sky and grass included, but the background will be blurred equally then of course.

[RichardSperry]

And I don't know how much time I should put into reading and understanding BetterSense's post.

Maybe someone can translate it.

The apparent depth of field in a photo changes depending on how large it's printed?

Counterintuitive.

[Steve Smith]

And I don't know how much time I should put into reading and understanding BetterSense's post.


The apparent depth of field in a photo changes depending on how large it's printed?

I think that's what he is claiming and I am suggesting otherwise.

I think depth of field is dependent upon the magnification at the film plane, not including the final print size.



Try this test (don't actually try it, just think about it).

Take two cameras, one 35mm and the other, 5" x 4"
Put a 'normal' focal length lens on each of them such that the subject is framed equally by both.
Set the aperture to f8 on both lenses.
Focus on the same place with both (e.g. a subject's eyes).
Take a photograph with both.
Print both up to 8" x 10"

Which one has the least depth of field?

Why?


Steve.

[Helen Bach]

And I don't know how much time I should put into reading and understanding BetterSense's post.

Maybe someone can translate it.

The apparent depth of field in a photo changes depending on how large it's printed?

Counterintuitive.

Strange, because I find it completely intuitive and, as BetterSense points out, quite obvious in practice. It's not necessarily 'how large it's printed', but more like 'how large it is on your retina' of course - this allows for both print size and viewing distance.

Have you never had the experience of seeing the difference in apparent sharpness (which is what DoF is all about) between a contact print and an 8x enlargement, for example? Or something that looks pin sharp all over at 8x10 but shows a limited DoF at 32x40?

If you disagree with BetterSense's post, doesn't that make it all the more worthwhile to try to read and understand it?

Best,
Helen

[Jim Jones]

At constant subject distance and print size, the only thing that affects the hyperfocal distance and DOF is the entrance pupil of the lens. Perhaps this will simplify the subject for those like me who don't handle formulae with the facility we did many decades ago.

[Brian Ellis]

Well you can define it however you want, but defining it that way is pretty useless unless the unenlarged negative is your final output. The only sensible definition is based on the print. Have you ever noticed that published tables for 'film CoC' are different for different formats? That's because the print CoC is the SAME. Despite your assertion, depth of field is defined by magnification of the FINAL PRINT. The fact that 'film CoC' tables have different values for different formats is a direct result of this.


Well if you are going to use a nonstandard and un-useful definition of depth-of-field, make sure everyone knows. The only person in the world who might care about DOF of the negative is the printer...and I don't even know why he would care.

In terms of thin-lens optics, format size is irrelevant to DOF. In real life, the results can be affected by diffraction, since the apertures used in enlarging/processing small formats may introduce more diffraction than larger ones used for larger formats. . .

I would think that anyone who makes a print would care about the negative. That's what you make the print from.

The size of the circles of confusion in the negative is one factor that determines how large the print can be made and still appear acceptably sharp (recognizing that different people have different standards of what's "acceptably sharp"). So for anyone who plans to print from a negative the size of the circles of confusion in the negative is important. Other things also affect "acceptable sharpness" such as the print magnification factor as you point out, viewing distance, the viewer's visual acuity, lighting in the display area, etc. etc. But that doesn't mean we lump them all together under the rubric "depth of field."

Diffraction has nothing to do with depth of field, nor does the aperture chosen for the enlarger lens (I guess that's the aperture you're talking about when you refer to "aperture used in enlarging/processing"). Both relate to the overall "sharpness" of the print, not to depth of field as such. See above paragraph.

With respect to your novel idea that only magnification and "aperture" affect depth of field, I'd suggest that you do some reading. Ansel Adams book "The Camera," at pp. 48-50, would be a good place to start. He discusses the three factors that affect depth of field (camera lens focal length, aperture, and distance from subject) and then discusses other things such as your favorite, i.e. print magnification, that all together determine how "sharp" a print is perceived to be.