Re: Lens shading/vignetting question

On Apr 17, 9:13 pm, ImageAnalyst <imageanal...@xxxxxxxxxxxxxx> wrote:
We put a lens on a big CCD chip and noticed a huge (2-to-1) drop off
in intensity near the edges, like dark vertical bands near the right
and left edges - not so much on the top and bottom edges.  Now if I
remember my terminilogy correctly "vignetting" happens when the
optical stop actually occludes part of the optical path, which I don't
think was the case here (it happened even with iris wide open).  (By
the way, there was no shading at all with the lens removed so it is
due to the lens.)  The manufacturer calls it "lens shading" rather
than "vignetting" which I agree with.  Would that be the more accurate
terminology?  What causes this shading?  Is it that there is more
glass to go through when the ray of light is traveling towards the
edge of the image and so it gets attenuated more, and so it's darker
there?  Is that the right explanation, or is it something else?
Someone said try bigger 45 mm machine vision lenses rather than
standard lenses meant for 35 mm sensors to reduce the lens shading.
Anyone agree with that?

Not sure if this is what's happening in your situation, but a lens
usually exhibits a roughly cos^4 falloff in intensity as you move
outward from the center of the image. A reasonable explanation of
that can be found here

doug.kerr.home.att.net/pumpkin/Cosine_Fourth_Falloff.pdf

Using a lens designed for a larger format would seem to be a way to
improve the uniformity, in light of that explanation, however I would
expect you would probably collect less light onto the sensor.

.

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