DOF on 8x10 vs. 4x5?

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Originally Posted by Doremus Scudder

I think this is fun.

We've got statements that range from, "format has nothing to do with depth of field," and, "the larger the film, the less depth-of-field for a given angle of view."

The interesting thing is, both are correct. The real players here are aperture and magnification.

I like to think about it this way: If we take a lens, say a 240mm lens with lots of image circle and make a photo from a given position on an 8x10 camera at, say, f/45, the depth-of-field will be X. If we swap out the 8x10 camera for a 4x5 camera, but keep the same lens, camera position, aperture and subject, and make another photograph, the depth-of-field will also be X. It's just that we now have a photograph with different borders; i.e., cropped in comparison to the larger photo. If we took our scissors and snipped out the 4x5 piece from the 8x10 film that corresponds to the image on the 4x5 film, we would have two identical images with exactly the same depth of field.

However, let's say we want to make a photograph with the same angle of view, i.e., the same borders, as the one we made on 8x10 film with a 240mm lens. Well, then we'll need a 120mm lens. The resulting image will have the same content as the larger one, but it'll be smaller. If we use the same aperture as the larger photo was made with, we'll have a lot more depth-of-field, however. Less magnification = greater depth-of-field. Or, we can duplicate the depth-of-field in the larger photograph by using a larger aperture; f/22 in this case. Turning this around, one can see that a smaller format often makes it easier to obtain more depth-of-field for a given image. It's simply that the image on the smaller format is smaller, i.e., less magnified and the depth-of-field is, therefore, larger. If depth-of-field is what you're after, a good guideline is to use the smallest format that will give you the image quality you need at the enlargement factor you desire. Going larger just makes getting the desired depth-of-field harder.

And, conversely, more magnification = less depth-of-field. Racking your bellows out for close-up work is similar to moving a projector farther from the screen: the image gets larger (more magnification). It follows then, that depth-of-field for any given lens/aperture combination decreases with bellows extension. Or, looking at it another way, the closer the subject is to the lens, the less depth-of-field we get from a given lens/aperture combination. That's why smaller apertures get used a lot for close-up-work and why arranging your subject so you can judiciously use camera movements to get the plane of sharp focus exactly where you want it is so important.

Best,

Doremus

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It depends on where you end your analysis.

Taking this a step further to the print:

If you contact print both, the 4x5 and 8x10 will have different depth of field.
If you enlarge both to say 32x40, you're enlarging the 4x5 twice as much as the 8x10, and you'll see the depth of field is the same. :cool:

Another step further to different viewing distances...

Then there's grain, aberration, tonality differences with magnification...

The psychological perception of depth in a print introduces a whole other set of variables, which might unnerve someone trying to reduce everything to a math equation, but to someone like me represents just more enticing options in composition. Sure you can make everything the "same"; but it won't "feel" the same. You might be able to quantify that, you might not. I always amazed me how some so-called art expert couldn't tell a stylistically "correct" studied fake Van Gogh, or Pollock, or Picasso from the real deal without chemical analysis, etc. But some of these fakes to me just didn't "feel" right - you just see technique with no genius in them. A real Van Gogh almost instantly floors you - he put his whole soul into every single brushstroke; maybe that's what drove him mad, over-the-top. Dunno. I do know when a given subject just "feels" right in one given format and not another, or from exactly one particular perspective versus another. Something just falls into place on the groundglass, and even four inches to the left or right it doesn't. I thrive on that kind of cat and mouse game.

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Originally Posted by alan_b

It depends on where you end your analysis.

Taking this a step further to the print:

If you contact print both, the 4x5 and 8x10 will have different depth of field.
If you enlarge both to say 32x40, you're enlarging the 4x5 twice as much as the 8x10, and you'll see the depth of field is the same. :cool:

Another step further to different viewing distances...

Then there's grain, aberration, tonality differences with magnification...

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Doremus is correct.

If you are talking about a 4x5 shot with a 120mm lens at f/45 and on 8x10 with a 240mm lens at f/45, enlarged to the same size, no they will not have the same depth of field. If the 120mm lens was at f/22 they would (this ignores any differences in field curvature or other optical differences).

Viewing distance is an irrelevant thing. Not once have a gone to a gallery where the viewer was locked into one position. I have seen people view images with a loupe or magnifying glass.

Also I think folks forget that DOF is a bit of a mushy term. There isn't "in focus" and then immediately "out of focus." It's a range, based on some amount of "acceptably in focus." The reality is that only one point in space is truly in focus.

Drew is absolutely correct on this.

Perception of what is focused what is not focused and the personality of what is out of focus-vs-in focus is extremely complex. This is where I'll take absolute issue of oversimplifying this extremely complex perception by math only.. the math regardless of how much or how it has been applied CANNOT properly describe the emotional and visceral effects of image perception as this is NOT a numeric or equation expression language alone. At best the math can be used as part of the overall description of focus perception.

All this is why the very best design folks use a LOT more than just mathematical analysis.

The other most important aspect is learning the language of expression. Those versed in Math as a language can gain much of what is being expressed easily and rapidly in much the same way as one who is versed in music, visual arts, and much more.

~Challenge is learning these languages of expression and how they are used as a means of expression.~



Bernice

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Originally Posted by Drew Wiley

The psychological perception of depth in a print introduces a whole other set of variables, which might unnerve someone trying to reduce everything to a math equation, but to someone like me represents just more enticing options in composition. Sure you can make everything the "same"; but it won't "feel" the same. You might be able to quantify that, you might not. I always amazed me how some so-called art expert couldn't tell a stylistically "correct" studied fake Van Gogh, or Pollock, or Picasso from the real deal without chemical analysis, etc. But some of these fakes to me just didn't "feel" right - you just see technique with no genius in them. A real Van Gogh almost instantly floors you - he put his whole soul into every single brushstroke; maybe that's what drove him mad, over-the-top. Dunno. I do know when a given subject just "feels" right in one given format and not another, or from exactly one particular perspective versus another. Something just falls into place on the groundglass, and even four inches to the left or right it doesn't. I thrive on that kind of cat and mouse game.

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Originally Posted by Corran

Doremus is correct.

If you are talking about a 4x5 shot with a 120mm lens at f/45 and on 8x10 with a 240mm lens at f/45, enlarged to the same size, no they will not have the same depth of field. If the 120mm lens was at f/22 they would (this ignores any differences in field curvature or other optical differences).

Viewing distance is an irrelevant thing. Not once have a gone to a gallery where the viewer was locked into one position. I have seen people view images with a loupe or magnifying glass.

Also I think folks forget that DOF is a bit of a mushy term. There isn't "in focus" and then immediately "out of focus." It's a range, based on some amount of "acceptably in focus." The reality is that only one point in space is truly in focus.

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I wasn't disagreeing with Doremus. He outlined both cases correctly: same aperture and different aperture between 8x10 & 4x5. I was attempting to add the context that perception of different formats doesn't end at the negative, as Drew and Bernice more eloquently elaborated.

Viewing distance is certainly relevant to one's perception of a work. The experience of getting up close to a small print or book is very different to walking up to a wall-size print.

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Originally Posted by Bernice Loui

Drew is absolutely correct on this.

Perception of what is focused what is not focused and the personality of what is out of focus-vs-in focus is extremely complex. This is where I'll take absolute issue of oversimplifying this extremely complex perception by math only.. the math regardless of how much or how it has been applied CANNOT properly describe the emotional and visceral effects of image perception as this is NOT a numeric or equation expression language alone. At best the math can be used as part of the overall description of focus perception.

All this is why the very best design folks use a LOT more than just mathematical analysis.

The other most important aspect is learning the language of expression. Those versed in Math as a language can gain much of what is being expressed easily and rapidly in much the same way as one who is versed in music, visual arts, and much more.

~Challenge is learning these languages of expression and how they are used as a means of expression.~



Bernice

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Bernice,

I couldn't agree more. The term, "depth-of-field" is imprecise; it depends on what parameters are used to define it, which may or may not have a direct relationship to the impact of the image.

Nevertheless, the physics of optics are based on natural laws and don't change, so the discussion about how "depth-of-field" changes with aperture and focal length (and magnification and viewing distance, etc.) is still valid. It's just that once we understand how all that works, we still aren't much more enlightened about the expressive elements of a photograph. That's a different discussion, to which the science of optics applies more loosely. I don't consider, however, that examining the physical properties of how light works is an oversimplification as long as one remains aware of this. Within its limited sphere, physics can get pretty complicated :)

Best,

Doremus

On a very cold quiet day at the National Gallery, with almost nobody else around, I spent over three hours nose-up to a Vermeer. What's amazing about his work, as well as DaVinci's to some extent, is that he studied, and attempted to replicate in his own paintings, how the seeming "flaws" in actual human vision subtly contribute to our sensation and emotional perception of the world. We aren't perfect optical systems ourselves, and that's part of the beauty of it! There's a lot going on subliminally. Nuances (I'm certainly no soft-focus type). Few current trends disconcert me more than seeing some highly stitched digital scene involving the assembly of many short focal length shots into an immaculately sharp big image clear across the board. And ironically, this doesn't consistently add to a sense of depth of field, but interferes with it! Too much applied technique with too little real perception. It's like looking at wallpaper. That's why I spend way more time just looking through a big groundglass and assessing the feel, rather than actually shooting. The hunt is just as important as the kill. I want to feel what details should draw the eye into acute focus, and what portions should subtly withdraw themselves a bit. It's all about depth of field being strategized esthetically, for overall compositional purposes. And this should not be confused with ideas like shallow selective focus to enhance bokeh, as is often done with telephoto lenses. That's fine in certain cases; but with the kind of plane of focus and perspective controls we have with view cameras, plus such a huge selection of potential lenses, more sophisticated controls are possible. Nor does it mean I have ignored the science; rather, I regard that as just the carrots in a much more complex, and hopefully more tasty, stew.

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Originally Posted by Drew Wiley

... Nor does it mean I have ignored the science; rather, I regard that as just the carrots in a much more complex, and hopefully more tasty, stew.

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Well put!

I hope one wouldn't just ignore the physics behind this stuff. It's useful if you just tailor it to your needs. You can feel free to change those guidelines to suit your preferences, your equipment, and your output methods.

The circle of confusion, other than often living up the name "confusion," is just a variable to define your system's tolerance for sharpness. Nothing more. If your film is huge and you contact print something to hang up above a mantlepiece where you can't get very close, your CoC can be large. If you enlarge massively from an APS sensor and view the print from nose-distance, it will be tiny. In practice, you just do the best, finest work you can for the negative most of the time and, once in a long while, you have a weird situation where you try to stretch things to the limit and maybe you use this theoretical stuff. That is when you might be glad you paid attention and figured it out for yourself.

Nobody is ignoring the physics. But a lot of people ignore that it's a pair of eyes that are final arbiter of such things. What are you trying to achieve? If you want a 30X40 inch print, that's around 4X linear magnification from 8x10 film, 8X from 4x5. Even I can understand that math. You likewise need twice the focal length of lens for 8X10 use as for 4X5 if you want that identical perspective. But that means you have to stop things down twice as much to get the same depth of field (not factoring movements); in other words, if you used f/22 for 4x5 with an 180mm lens, you'd need f/45 for a 360 to obtain the same perspective and depth of field. Factors of two. I frankly don't give a damn about "circles of confusion" and "normal viewing distance"; and hyperfocal theory comes into my head only about twice a year when doing MF work. Nor do I pack a calculator. A simple magnifying loupe tells me everything I really need to know. I have a close friend who is a famous astrophysicist specializing in the physics of light, but he's never been able to take a decent picture with an ordinary camera.