not trying to argue here, i'm just a little confused:
the 1.5 factor comes into play because the size of the sensor is different than the film plane on a 35mm camera, as the film plane on a mamiya is different than a 35mm format camera. Essentially, the coverage and angle of view on a 80mm mamiya lens is vastly different than the angle of view and coverage of an 80mm lens for a 35mm format camera. On the 645 format, an 80mm lens is a normal perspective lens, on a 35mm format an 80mm lens is a telephoto. Are you saying that the 80mm mamiya lens on a dslr sensor is going to be 120mm? My math is coming from frame diagonal ratios Further, how invalidated would the focusing ring indicators be?
my math:
80mm (645 lens):75mm film diagonal AS Xmm (35mm format lens):35mm film diagonal, then multiply by 1.5 to get
the dslr equivalent...this gives me about 70mm for the 80mm mamiya on a dslr (it does, indeed, become a telephoto lens), and 24.5mm for the 35mm mamiya lens on the dslr
David Dyer-Bennet <dd-b@xxxxxxxx> wrote:
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
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