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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