Perspective compression is a result of the ratio of the focal length to the
distance(s) of the subject(s) being photographed. If you take two pictures,
one with a dslr and one with a film slr, and do not change the
camera-to-subject distance, the perspective compression will not change.
The framing will, however be different--the dslr image will be a cropped
version of the film slr image. If you move the camera to make the framing
of the two images similar, the perspective compression will be different.
Depth of field works basically the same way with one little twist. Same
lens, same aperture, and same camera-subject positioning yield the same
amount of out-of-focus fuzziness recorded by the camera regardless of
whether you record with 35mm film or a 1.5x cropped digital sensor. The
little twist is that the depth of field in the final print depends on amount
of magnification used to create it. If you create an 8x12 print from the
35mm film image and compare it to a 5x7 print created from the dslr, the
magnification is about the same and thus the apparent depth of field will be
about the same. As with the depth of field, moving the camera to create two
similar images yields different geometries. If you then create two final
prints that are the same size, you have also magnified the recorded images
differently.
So it really depends of the details of how you take the pictures and then
create the final prints. Depending on the those details, the correct answer
can be either 1) or 2).
Hope this helps,
Mark
Russell Kerstetter wrote:
I mostly lurk here, however I do have a question that I have had
answered is two different ways, and always by someone who knows more
than me.
The Pentax dslr's have a 1.5 crop factor, so this means either:
1) A 50mm focal length is still a normal perspective only with the
sides cut off, because the sensor is smaller than a 35mm negative, and
so the recorded image is simply smaller because of that.
....or....
2) A 50mm focal length is now a medium telephoto perspective, with the
same effects, with regard to image compression (from front to back)
and depth of field, in the same way that on a large format camera a
50mm focal length is a wide perspective.
So....
I feel like there is some concept about the mechanics of a lens that I
do not understand, but both explanations make sense to me, so I don't
know.
Russell
