At 10:54 11/28/02, Joe Gwinn wrote:
I was thinking also of the lenses used on view cameras, where the field of
view (that is, image circle) vastly exceeds the film size. Ditto, shift
lenses for 35mm. So, I was looking for an algorithm that included only
data that was true of all lenses, regardless of design, which leaves only
focal length.
Cannot be done. A practical lens design has an "acceptance angle"
independent of focal length which is the limiting factor for the FOV of the
lens' _image_circle_. You are correct that technical camera lenses
normally have an image circle diameter significantly larger than the film
diagonal to allow for the adjustments moving the circle around across the
film plane. It also means the 80mm Mamiya Sekor acceptance angle for an
M645 must be much larger than the 85mm Zuiko acceptance angle for an OM.
However, when one speaks (or writes) of a system FOV, the limiting factor
is the film dimension, if it's properly designed with the lens' image
circle being larger (even if slightly more) than the film dimension. Even
in 35mm small format and medium format systems, the image circle is usually
a little larger than the film gate, especially for shorter
lenses. Reason? It mitigates cos^4 falloff in the corners by placing the
bulk of this falloff outside the film gate. The tradeoff is the extra
light that doesn't end up in the photograph potentially bouncing around
inside the lens and reducing contrast, or worse yet causing aperture flare.
-- John
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