At 11:38 10/7/00 , Andrew Fidles wrote:
>
>Hmmmm. How then do you explain the size of the 80/2.8 Planar on a
>Rolleiflex? A lot of quite fast lenses on folders were TINY compared to
>35mm lenses of equivalent speed. Surely the lack of focussing movements
>can't explain it all, especially given the built in leaf shutter.
>Andrew
Yes, and a couple of other very small lenses are the Rollei 35S f/2.8
Sonnar, as is the original 50/1.5 Sonnar for the ZI Contax (focus helical
on body, not lens).
There are other design considerations (or goals) which muddy up the waters
on lens objective diameter versus its speed. One of them is minimum focus
distance. Another is how much bigger the image circle diameter is than the
film gate diagonal.
Minimum focus for both of the above Sonnar lenses is 3 feet. The minimum
on the Sekkor I mentioned is about 2 feet, and going from 3 to 2 feet
extends the lens considerably further. Minimum focus distance is a very
real problem in making a fast macro lens that focus to infinity also.
Classical close focusing by continuing to move the lens farther away from
the film plane does reduce the light. The image circle grows considerably
at very close distances, as is the extension required. It's the reason an
extension tube reduces effective aperture. I haven't seen many older
(vintage) cameras that get below 3 feet with a "standard" lens.
Cos^4 falloff is another issue, especially for shorter focal length lenses.
One of the standard methods for reducing it is designing a larger image
circle to put the falloff outside the corners of the film gate. I don't
think the older cameras (i.e. the folders) did this as much and you see
more falloff in their lens formulations. Many, but not all, older
(vintage) camera lenses do exhibit more falloff leading me to believe at
least a sizeable portion of it is due to an image circle diameter closer to
the film gate diagonal.
I don't know what the minimum focus distance is on the Rolleiflex and its
f/2.8 Planar, but it must be somewhere between 3-4 feet? Most older
cameras do not focus "standard" lenses below that.
-- John
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