Bar-Bet: Does DOF change with format?

[Jeff Conrad]

Dan, things change a lot with closeups--the differences between formats aren't as great, so what you're saying is exactly what I would expect. Again, the common rule of thumb relies on quite a few assumptions; if they aren't met, all bets are off.

So really, you and Paul are probably both right--you're just assuming different conditions.

[Jack Dahlgren]

What baffles me is that it is absolutely, 100%, beat you over the head obvious that, in practice, the DoF when using LF is vastly less then for 35mm. Anyone who has shot any number of pictures at all will easily observe this. I suppose one could argue that LF has the same DoF, it just requires a lot more light. Now that is in practice. One can mathematically masturbate till the cows to come home to show all sorts of other conclusions (some of them even interesting), but it is all just that; masturbation. If you are concerned about outcomes, then it is clear.

Of course, when it comes to bar-bets, it pretty much depends on wankery, or else it wouldn't be a bet would it?

Try this thought experiment - or do it for real. Instead of a sheet of film, tape a 35mm frame of film in a film holder and expose it. The image is exactly the same without regard to the format of the film right? Suppose that small image is exactly what I want to capture. Now how do I do that while keeping everything else the same? Well the only thing I can do is move in much closer so the subject fills more of the frame.

The reason it seems that dof is less with LF "in practice" is that you are closer to the subject when using it. My small format 135mm lens doesn't even focus closer than a couple of meters, yet that is a typical shooting distance for a 135mm on large format. See what sort of depth of field you get with a 300 mm lens on small format a few feet away from the subject.

[Paul Kierstead]

Jack, the whole point of in practice is that it isn't an experiment or contrived example. And it isn't picking a lens length on two formats and comparing them (unless, I suppose, your job is lens testing or something).

I'll give you this thought experiment: Got out and take some head and shoulders portraits on 35mm. Use appropiate lenses to get a natural look. Now do the same with 4x5: Go shoot some head and shoulder portraits with a natural look. How is the DoF going to compare? Now that is in practice.

[sgelb]

Ok: This is a little like a bar-bet though it came up over coffee. We were discussing portraiture. I shoot LF and my friend (a professional wedding photographer) shoots with a top-of-the-line DSLR, (Nikon D-3).

My question: For the same focal length and same aperture; does DOF change with format? My thought was that 90mm at 5.6 would give nose-eyes-ears sharp and blur the background regardless of how large the film was.

My friend says that DOF will be shallower in LF for the same focal length and aperture.

For me to be right, the image size must also be the same . . .right?


Waitress: Two more over here please . . .thats right—De-Caff.

the larger the format, the shallower the DOF.

f/2.8 on 4x5 is like .95 or so on 35mm

[Robert Hughes]

the larger the format, the shallower the DOF.

f/2.8 on 4x5 is like .95 or so on 35mm

Wrong - you're mixing too many variables. A 150mm lens on 4x5 is exactly same DOF as 150mm on 35mm format, and with a 150mm lens, f/5.6, DOF is exactly the same for any format.

[rdenney]

Just out of curiosity, why do you multiply DOFMaster CoCs by four for 16x20? Two would seem to do (unless you always pull out your loupe).

Because 8 X 4 = 16, silly. (That was a joke; you can laugh. Of course, two is what I meant.)

Actually, the Zeiss article mentioned in another thread just filled my last hour, and provided a little more clarity on the terms, though I also found that my understanding was sufficient. It really should be that "circle of confusion" describes the diameter of the blurry spot, and not to the standard by which it is evaluated. That standard should probably be called the "critical circle of confusion diameter".

The Airy disk confusion seems forgivable: I always evaluated telescope performance the conventional way by looking at the diffraction rings with the telescope slightly defocused inside and outside, when doing a star test.

Rick "thankful not to have to revise his bokeh article after reading that Zeiss white paper" Denney

[rdenney]

Speaking of the Zeiss white paper linked in another thread, I thought I would link it here, despite that providing authoritative descriptions has little relevance to a bar bet. But here it is anyway:

Zeiss white paper on depth of field, depth of focus, and bokeh.

Rick "who has added this to his library" Denney

[Darin Boville]

Wrong - you're mixing too many variables. A 150mm lens on 4x5 is exactly same DOF as 150mm on 35mm format, and with a 150mm lens, f/5.6, DOF is exactly the same for any format.

Of course, you might want to make prints from the negs...

--Darin

[Jeff Conrad]

Because 8 X 4 = 16, silly. (That was a joke; you can laugh. Of course, two is what I meant.)

That standard should probably be called the "critical circle of confusion diameter".

Circle of confusion wears many hats, and terminology varies among sources. Blur spot due to defocus is arguably the best choice, because it isn't always circular (it's the shape of the aperture, sometimes distorted if off axis), and with tilt or swing, even a circular blur spot is elliptical. It's not possible to separate the blur spot due to defocus from that due to diffraction, so even this is a bit of a misnomer.

Most sources I've seen formally use something to the effect of acceptable circle of confusion diameter, sometimes omitting diameter, but generally soon fall back to just circle of confusion because the strictly correct term gets pretty unwieldy. Formally, I usually use something like defocus blur spot, but quickly go to something like CoC because anything more is just too much to read repeatedly. As long as it's made clear what's meant, there usually isn't a problem.

That said, it's sometimes also important to indicate whether the CoC (or whatever) applies to the initial image or the final image. It's usually clear from context, but I've seen some pretty heated arguments that derived simply from two different interpretations. Especially after a few pints ...

[Darin Boville]

>>whether the CoC (or whatever) applies to the initial image or the final image.<<

Exactly. Perhaps it is just my reading skills but that distinction has not been very clear at times throughout this thread...

In any event, to my mind it is the final image that should be the "standard" for judging depth or field or CoC since, presumably, we are talking about pictorial photography...I've seen very few negatives on display...

--Darin