trevor cunningham wrote:
My math is starting from the 645 film frame with a
75mm diagonal, not the 35mm diagonal. The 35mm mamiya
lens is the equivalent of a 16mm lens for 35mm format.
With the 1.5 ratio, that would be a 24mm lens on the
d200? Out and out, the lens IS a fisheye, or at least
close to one. The same math would make the 80mm
mamiya lens a 56mm lens on the d200, still a normal
lens, not a tele.
Focal length is a basic physical property of the lens system. A 35mm
lens brings parallel incoming light rays (from "infinity") together at a
point 35mm from the lens. (That's the "thin lens" approximation that
everybody learns.) The lens, of course, doesn't know what's at that
point -- ground glass? Film? A digital sensor? What size? The lens
doesn't know, it can't. The image being formed is constant. Different
size sensors (that fit within the image circle) will record images with
a different angle of view; but what they record will be cropped portions
of exactly the same image circle.
So; the field of view is the result of the operation of a lens of
particular focal length on a receptor (film/sensor/ground glass/etc.) of
a particular size.
We say that a 50mm lens has the field of view of a 75mm lens on a 1.5x
crop factor DSLR (like the Nikons). And it does. But it doesn't make
any difference at all (to this) what the mount on the back of that lens
is; it's exactly as true of a 50mm Mamiya lens intended for 645 as it is
of a 50mm Nikkor lens intended for 35mm.
You can't beat the physics; it always wins. (You also can't break even,
and you can't get out of the game, but that's a reference to a somewhat
different area of physics.)
--
David Dyer-Bennet, dd-b@xxxxxxxx; http://dd-b.net/dd-b
Pics: http://dd-b.net/dd-b/SnapshotAlbum, http://dd-b.net/photography/gallery
Dragaera: http://dragaera.info
