Perspective is controlled entirely by photographer-subject distance
(this can be proven by taking a shot at multiple focal lengths form the
same position, cropping down will show an identical perspective). The
crop factor only affects field of view, but since you would have to move
relative the subject to get the same composition from a digital as from
a film body, you will get perspective equivalent to the 75mm telephoto
on film, from a 50mm on digital.
Depth of Field is related primarily to focal length, so it will only
change slightly from film to digital (Digital requires smaller Circles
of Confusion than 35mm film, so you lose a slight amount of DoF,
acceptable CoC size is the only reason DoF varies between formats, the
larger the format, the larger the acceptable CoC).
So a 50mm on digital will have the same angle of view and perspective as
a 75mm on film, but the Depth of Field at a given aperture will be only
imperceptibly less than a 50mm on film.
-Adam
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
