An example may help illustrate these points. Let's choose two extremes--the 24x36 DSLR you are used to and, say, an 8x10 view camera. And let's say we are going to make a full-face portrait. And, to maintain simplicity, we'll limit ourselves to a "normal" lens with a focal length approximating the diameter of the image frame.
Let's start with the 8x10. If I make a full-face portrait on 8x10, that face may be, say, five inches tall on the film. The actual face may be 10 inches tall in real life, so my image on the film is a 2:1 magnification. With a 12-inch lens, that means the lens will be about 24 inches from the subject's face.
That photo is nearly in the macro range, and we all know that at macro magnifications, the depth of field is really narrow. According to DOFmaster, at f/9, the depth of field will extend from 1.99 feet to 2.01 feet.
Now, I could put a 300mm lens on my Canon 5D, and the magnification would be exactly the same. So would the depth of field. But here's the problems with doing that: 1.) I'd need a bunch of extension tubes to get that 300mm lens to focus, and 2.) I'd only be able to include the tip of the nose in the picture. I could back up the camera to include the whole face, but then it wouldn't be the same picture. So, I have to use a shorter lens.
With a 50mm (2-inch) lens, I can make pretty much the same picture. With the camera in the same position, the 2-inch lens will make an image on my 24x36 frame about 0.8 inches tall. That's a magnification of 1:24--not at all close to the macro range. To get similar depth of field as what I was able to get at 1:2 with the 8x10 camera and its 300mm f/9 lens, I need something a hair faster than f/1.0, which is currently not in Canon's lineup.
And if I used a 300/5.6 lens on my 8x10 camera, there is no lens made for 35mm that is fast enough to provide a similar selective focus effect.
Now that we have established incredibly narrow depth of field using the large format, we have another advantage for the stuff that IS in focus. A given print will be enlarged only 1/8 as much. So, if an 8x10 print is my goal, the 24x36 image will require an 8x enlargement, but I can make a contact print from the 8x10 negative.
Thus, a full-face portrait with 8x10 will have considerably less depth of field and therefore much greater selective focus (if that's what I want), but in the focus plane, it will provide far greater resolution and contrast on a given size print. The results border on the mystical. People try to simulate the effect using tilt lenses on their small cameras and the like, but it's just not the same.
I have an old Ilex 8-1/2" f/4.5 lens. It's a Tessar formula--nothing at all fancy by today's standards. It's slightly longer than normal on my 4x5 camera, which I like for portraits. Despite that it's an old hunk of junk, I keep it because of that f/4.5 aperture. At f/4.5, I can get an effect impossible with small format and very similar to my 8x10 camera with the f/9 lens. No, the lens isn't that sharp when used wide open, but I may only be enlarging it by a factor of 2, so it doesn't have to be.
To me, managing what is in focus and out of focus has a greater effect on the look of a photo than just about any other decision we make. The large-format cameras give us huge control over that decision, beyond what we can do with small format. Even when we want everything sharp (which, for me, is most of the time), getting it so with a small camera is often impossible because we'd need f/32 and, even assuming our lens goes down that far, we can't live with the fuzzy diffractive mess that results. But with careful camera tilts and swings, we can usually provide the sharpness we need where we need it, even considering that we have less depth of field. And it takes a mighty small aperture to ruin a large-format image with too much diffraction.
Rick "finding cameras without movements to be pretty limiting for landscape applications" Denney
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Originally Posted by bensyverson
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