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Re: [OM] Mind Bender (intermediate focal length?)

Subject: Re: [OM] Mind Bender (intermediate focal length?)
From: Thomas Bryhn <thomas.bryhn@xxxxxxxxxx>
Date: Tue, 03 Dec 2002 18:40:51 +0100
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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