Depth of field is affected by only two characteristics - f ratio and
reproduction ratio. The f ratio is obvious, small aperture = greater DoF.
Reproduction ratio, defined as the film image size divided by the actual
object size, is less so. Let's assume a 90mm lens on a 35mm body. The
actual object is 2.4 meters tall and in horizontal format fills the frame
top to bottom. So the size *on the film* is 24mm. This give a reproduction
ratio of 24mm/2400mm or a reproduction ratio of 1:100. Now, lets put on a
24mm wide angle lens. If the camera stays in the same position, then the
DoF is much greater, agreed. But the reproduction ratio is also very
different; the 2.4 meter tall object no longer fills the frame. To do that,
we have to move our camera position closer to the object. A lot closer.
Once we have the same reproduction ratio, we again have the exact same DoF
as with the 90mm lens. This principle applies regardless of film size or
focal length. Could just as easily be using an 8x10 format or a tiny CCD
sensor as the example.
So if reproduction ratios are the same and DoF is the same, what changed
then? The perspective! The wide angle lens will provide a much greater
size perspective between the foreground and the background than the moderate
90mm telephoto.
My source for this is a book I recently acquired on large format technique.
I've read and re-read this DoF discussion several times and finally tried it
using a ruler and my OM's. Made a believer out of me.
Terry and Tracey <foxcroft@xxxxxxxxxxxxxxxxxxxxxx> wrote:
>In a John Shaw book, he says a 90mm lens has the same DOF whether it is
>mounted on a 35mm, 6*6 or a 5 *4. I'm so confused.
in response to Bau Struye original post asking:
>>I read somewhere that since the digitalcameras use shorter lenses
(starting
>> around 6mm, I think), their depth of field is MUCH greater.
>> ...
>> >Can anyone comment on the depth of field differences between digital and
>film?
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