[Leica] 24 x 36mm ?

[henningw at archiphoto.com (Henning Wulff)]

At 8:51 PM +1000 6/8/07, G Hopkinson wrote:
>Frank, I'd never even considered a difference in image size from 
>different focal length lenses. Henning suggests that those from
>long teles are markedly different, too.
>I'm hazy on rear lens vertex, but I thought that the exit pupil will 
>always overlap the film gate, so it won't come into the
>calculation. I thought that irrespective of the entry angles at the 
>objective the exit angles were similar. They all must produce a
>sufficient exit pupil to more than cover the film gate. Any 
>difference in the exit angle would only be significant if the depth
>between shutter opening and film gate was significant?  Sounds like 
>a job for Erwin Puts!
>
>I've probably made a total dog's breakfast of that. Perhaps someone 
>will set me straight.
>
>Really my posts were about the size of the film gate with different 
>M models. From my samples here they do differ as I noted.
>
>I only have M lenses here, but my curiosity is sufficiently piqued 
>now to shoot a series of frames from 28 to 135 on the end of my
>current film, to see if I can detect any difference. The inquiring 
>hoppy wants to know!
>
>cheers

The rear exit pupil is the effective rear nodal point where all the 
rays last cross. By definition, a simple or fully symmetrical 21mm 
lens will have its rear nodal point, or exit pupil, located at 21mm 
from the film plane when the lens is focussed at infinity. Similarly, 
a 400mm non-tele lens, like the f/6.8 Telyt, will have its rear exit 
pupil at 400mm from the film plane. These numbers have nothing to do 
with the coverage that these lenses provide, although in this case 
both are designed for 35mm.

The 15mm Hologon f/8 has an exit pupil that is very close to 15mm 
from the film plane, and has a diameter that is about 2mm. The 
400/6.8 Telyt has an exit pupil that is almost 59mm in diameter.

Since the light rays from the Hologon came from a 'point' only 15mm 
from the film plane, and centered on the film aperture, they sneak 
under the edges of the film gate and thus produce a larger negative 
than the rays from the 400mm Telyt, which come to the film gate 
almost perpendicular to the film.

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    *            Henning J. Wulff
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