Bar-Bet: Does DOF change with format?

[Jeff Conrad]

Actually, that would be depth of focus. Depth of field applies in the object plane.

Depth of field and depth of focus are slightly different concepts. The former normally refers to the range of object distances that are imaged with acceptable sharpness on a fixed image plane; the latter normally refers to the range of image distances over which an object at a fixed distance is imaged with acceptable sharpness (e.g., film buckling or misfocus).

The two phenomena are approximately conjugate; however, the depth of focus is symmetrical about the image plane, while this is only approximately true for the image distances corresponding to the limits of depth of field.

[Darin Boville]

does an image have DoF if no one can see it?

If no one sees an image not only does it not have depth-of-field it doesn't even exist as an image! That fact that the image obviously continues to exist in the dark, and that the inks/dyes/grains of silver are still rearranged in such a way to cause "depth-of-field" even with the lights out, merely reveals that the image is at all times being viewed by a power higher than that of man.

Depth-of-field illustrates--indeed demonstrates--the existence of God.

--Darin

[John Bowen]

Whenever I'm at a bar and switch formats, say from lite beer to single malt scotch, my DoF decreases and my Circle of Confusion increases....

Sorry, I couldn't help myself and besides this thread needed some hummor....

[GPS]

If no one sees an image not only does it not have depth-of-field it doesn't even exist as an image! ...

--Darin

Not true! If you bring a group of blind people to a gallery, none of them sees any image. Yet the images do not stop existing, they are still there with their depth of field...

[Jeff Conrad]

Do you use the same glass for both formats?

[GPS]

I think that's clear enough that the bet concerns the same glass of beer for both formats.

[rdenney]

I'm probably an extremist ... I think any comparison such as we're talking about is necessarily contrived.

Which was my point.

But if I wanted a particular image, I'd probably choose the same camera position regardless of format.

Hmmm. When I walk around with a small camera, I use the camera as a viewing card, looking for compositions that please me. When I see one, I push the button. That "viewing card" has a much wider range of options available to me than does my large-format camera.

When using a large-format camera, I walk around without the camera, using my mind as the viewing card. Given that I'm photographing using the large-format camera, the range of possibilities is already built into my thinking, so I automatically edit out image opportunities for which I have no lens.

Also, I have different objectives when I use large-format. Images I would reject with smaller cameras because of their lack of image-management tools I can attempt with a view camera. And the image-management tools and format characteristics of the view camera lead me to explore images that highlight what those tools and characteristics provide. I have made images on small cameras that would be impossible on a large camera, and vice-versa. Knowing those differences informs our image-selection process whether we do it on purpose or not.

I suspect few select their subjects, compositions, and tonal visualizations without any consideration for the equipment at hand.

If I had a particular composition in mind before leaving home, then I would bring the equipment that would help me realize that visualization. That is not always the big camera, of course. But that happens rarely with me--maybe I see an image or a fragment of an image while driving by or while exploring, and come back later. But even then, the details of my visualization will often migrate to the capabilities of the equipment I brought.

But the real point is that we tend to set extreme examples for our comparisons: Either no enlargement or enlargement to the same print size. I suspect most do something in between, which just emphasizes how much those extreme cases are contrived.

Assumptions are a model of the conditions, and, as they say (maybe not in bars), all models are false, but some are useful.

Rick "allowing alternative explanations" Denney

[Jeff Conrad]

I usually using a real viewing card regardless of format so that sloth doesn't have undue influence on the camera position. Ansel would be proud of me ...

But the real point is that we tend to set extreme examples for our comparisons: Either no enlargement or enlargement to the same print size. I suspect most do something in between, which just emphasizes how much those extreme cases are contrived.

To be honest, I don't spend a lot of time thinking about comparisons. I usually carry only one format at a time, and I get whatever DoF I get (as I said, I usually change the f-number to suit). But if a comparison, however contrived, is to be made, we need to state the conditions; they don't necessarily need to be my "same picture" criterion. That's really all this discussion is about.

Maybe the real answer is to go easy on the single malts and avoid the bets ... or at least avoid the bets.

[Paul Fitzgerald]

"My question: For the same focal length and same aperture; does DOF change with format?"

Yes.

IF the same FoV is used the smaller format wins,
IF they are enlarged to the same size the larger format wins.
Basically it's a wash, buy another round and watch the girls go by.

Actually the DoF is only controlled by 2 factors, apparent aperture size and front focus distance. Why? Because all prints are assumed to be enlarger to the same size which adjusts for magnification, enlargements and CoC.

[Paul Kierstead]

Heh, you may assume that but this thread makes it clear that not all have the same assumptions