Bokeh between 8x10 and 4x5

[Brian Ellis]

OK I may be wrong but I always thought that if you frame the same (i.e. same size image), the DOF is roughly (proportion in front to behind the exact focus in acceptable focus may change slightly) the same, independent of focal length of lens? This can be seen from DOF formulas, which have the f-stop in the formula, but not the focal length (or the product of aperture AND the focal length, which is the f-stop number). There is a common misconception that shorter focal length lenses have greater depth of field but if you even up the image size, that's just not the case. Not sure what this has to do with bokeh (signature rendition of the out of focus areas), though!
OK ket the arguments begin!

If you change the focal length of the lens but frame the same then you necessarily have also changed your camera position, i.e. you've moved closer to or farther from the subject. Since distance from subject is one of the three factors that affect depth of field together with lens focal length and aperture, you've changed two of the three factors, not just one, and you've changed them in opposite ways. E.g. if you make a photograph with a 300mm lens, and then make the same photograph (as nearly as possible) with a 150mm lens, the shorter lens produces greater depth of field but you have to move closer to the subject to make the same photograph, which decreases depth of field. Ken Lee gave the relative proportions of each change in his message.

Shorter focal length lenses do produce greater depth of field than longer lenses if all you change is focal length, i.e. same aperture, same camera position, same enlargement factor, etc. etc. That isn't a misconception. The key to your misconception statement is that you said that in addition to using a shorter focal length lens you also "even up image size," which means you haven't left everything except focal length the same, i.e. to "even up the image size" you've necessarily either changed camera position when you made the photograph or changed the magnification factor when you made the print.

[Richard K.]

Shorter focal length lenses do produce greater depth of field than longer lenses if all you change is focal length, i.e. same aperture, same camera position, same enlargement factor, etc. etc. That isn't a misconception..

Well sure, but the image size is way smaller, objects in front and in back of the main focus are smaller and the COF is smaller, so yes there is more DOF BUT it's a different photo! I think that the misconception part is that for an identical composition (main subject image size the same) the shorter focal length lens will have greater DOF for a given f-stop. I'll concede that because of the size difference of objects behind and in front of the main subject, the proportion of the DOF in front and back changes but not the whole DOF - talking about regular (approximately infinity) photographs, not macros! I guess what I thought I was replying to was "if I take the same photo (same crop) in 4x5 and 8x10 at the same f-stop, with 150mm and 300 mm lenses (yes, I'm twice as far away now) respectively, will there be greater depth of field on the 4x5?" (If that wasn't the OP's question, then I'm not answering what I thought I was.)

I think it will be the same. Am I wrong? I guess what I think I'm saying is that DOF depends on f-number not focal length (directly) if the image size is the same, to a very good approximation. I'd be totally happy to be told I'm wrong by someone other than my wife and teenage sons!
Here is a simplified demo:

http://www.bluesky-web.com/dofmyth.htm

[Richard K.]

Please, I am not trying to be argumentative ...

Alan, me neither! We're just trying to learn, right? But there is a difference in hyperfocal distance formula (which clealy needs to factor in focal length) and DOF formula. DOF I think (see I'm not sure ) depends only on f-stop and image size for usual near infinity (which can be as close as 20 feet!) photography. Please see my reply to Brian Ellis ...

[sparq]

Richard, yes, you are wrong. Read Brian's post again, he gave you a good explanation why. You mix together DOF effects of varying FOV for a fixed format with DOF effects of varying format for fixed FOV - in both the focal length changes but the former one is irrelevant in the context of this discussion because, as you correctly said, you get a different picture.

Ad your last post: How could DOF stay the same while hyperfocal distance increases? Think of it, that makes no sense. There's no magic there, just simple geometry.

[Richard K.]

Ooops, with 300mm lens on 8x10, I'm obviously the same distance away as 150mm on 4x5! However, I don't think there will be a difference in DOF but please explain if I'm wrong. But I'll stand by what I thought was being argued and just state that for a given format and a given subject framing and aperture, lenses of all focal lengths have exactly the same depth of field (yes the distance to the subject would have to change to keep the subject size the same). Wrong or right?

[Richard K.]

Richard, yes, you are wrong.

Yes, I misunderstood, I am wrong. I'll go back to things I know about...like spending money on more LF stuff!

[aduncanson]

Yes, I misunderstood, I am wrong. I'll go back to things I know about...like spending money on more LF stuff!

But don't be too hard on yourself. I just ran through the math and confirmed that your point is essentially true for the case where the f-stop, format, circle of confusion and image size (or subject distance to focal length ratio) all stay the same - as long as the subject distance is relatively short, say less than about 10 focal lengths. Changing to a shorter focal length lens gives very little increase in depth of field.

However, as the subject distance to focal length ratio becomes larger, the shorter focal length does increase depth of field significantly.

I did not know that.

[Ken Lee]

I should have mentioned the formula I suggested, I got on this forum, from Leonard Evens: one of our esteemed LF members, Professor Emeritus of Mathematics at Northwestern University, and all-around nice guy. It may be a rule of thumb, but it's simple enough that even I... can grasp it.

Here is an example that gives you a sense of the depth of field you get with a variety of focal lengths, each of which is a "normal" lens for 6x6, 4x5, and 8x10 respectively.

75mm lens at f/16 = 150mm lens at f/32 = 300mm lens at f/64

[Frank Petronio]

Well it's easy and cheap to find a 300/5.6 for an 8x10 but those 150/2.8 Xenotars for 4x5 are expensive... so the practical answer to his question is to just get an 8x10.

[Alan Davenport]

An assumption is made in depth of field equations, namely that we are making a "standard" 8x10 print, and that it will be viewed at a "normal" viewing distance. Since the OP is changing both focal length and film format, we also have to assume a different CoC.

A 150mm lens on 4x5 film will have the same angle of view as will a 300mm lens on 8x10 film. Since the 8x10 film will require only half as much enlargement to make any given print size, it can tolerate a CoC twice that of 4x5 film. The resulting depth of field in a print will be half as much with the 300/8x10 as with the 150/4x5.

Example: A subject 5 meters away, shot with a 150mm lens at f/5.6, on 4x5 film, and assuming a CoC of .1mm, will produce a depth of field of roughly 1.24 meters (in the subject space.) An 8x10 camera with a 300mm lens at f/5.6, because it will tolerate a .2mm CoC, will have about 0.59 meters DoF in the subject space. Thus the answer to the OP's question is one half as much depth of field with the 8x10.