At 00:33 03.12.02, Joh A. Lind wrote:
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.
I'm no expert optical engineer, but this "reason" doesn't make any sense.
You can't just "place the bulk of the falloff outside the filmgate", the
cos^4 is there from pure geometry (and the simple lens approximation), and
it can't be moved around. It's a simple monotonous function in the region
that the equation applies (0 to 90 degrees). For instance, one of the four
cos terms comes from the fact that if a person looks straight into a
circular opening (=a lens) of area A, and then moves an angle x to the
side, the opening will no longer appear as a circle but as an ellipse of
area A*cos(x).
True, there are lens designs that have better falloff characteristics than
the simple lens approximation implies, but for any given lens design or
geometry the falloff can't be reduced by introducing mechanical vignetting
(= making the image circle smaller). Just think of an OM lens mounted on a
halfframe body, the falloff inside the halfframe is exactly the same even
if you stack 4 or 5 filters to vignette the parts of the image circle
outside the halfframe image. I could even argue that masking a full 35mm
frame down to halfframe with black tape before taking the picture is
vignetting, just at a different position in the optical path, and that
wouldn't affect the unmasked part of the frame, would it?
I venture to say that contrast is the only reason (besides costs of larger
lens elements) why lenses are designed to vignette and thus always having
an image circle just slightly larger than the film/sensor diagonal. And a
good reason it is.
Thomas Bryhn
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