Re: Lens shading/vignetting question
- From: ImageAnalyst <imageanalyst@xxxxxxxxxxxxxx>
- Date: Thu, 24 Apr 2008 16:39:15 -0700 (PDT)
On Apr 18, 1:39 am, "anorton" <anor...@xxxxxxxxxxxxxxxxxxxxxxxx>
wrote:
"ImageAnalyst" <imageanal...@xxxxxxxxxxxxxx> wrote in message
news:dd2ae235-9b81-4570-9166-5a17ba8360c9@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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?
Vignetting does not occur when the aperture stop occludes the optical path..
The stop will ALWAYS occlude the path (otherwise it ain't an aperture stop).
Vignetting occurs when some other edge partially or fully blocks the path
for some parts of the field but not others. You will actually see the most
vignetting (if there is any in your image) when the iris is open, and the
least when it is closed.
Any lens designed for a certain size format will likely show significant
vignetting when used with a larger format. Aside from that, it will also
likely have really large aberrations outside the designed format.
Usually the vignetting appears circular since the internal components are
usually circular. The vertical bands on the sides and the term "lens
shading" used by the manufacturer make me think this lens has a rectangular
sun shade on the front of it that is doing much of the vignetting.
Adam Norton
Norton Engineered Optics
Optical design and systems engineering for Silicon Valley and beyond.www.nortonoptics.com
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Thanks to all 3 who replied. The dark bands on the side appear
somewhat worse than they really are because the image has a 4:3 aspect
ratio so the image extends farther out to the sides than vertically.
The attenuation in the vertical direction at the same distance from
the center I think was roughly the same. I think we'll take the
advice and try a lens made for a larger format. We have plenty of
light so any additional attenuation due to the lens having more glass
would not be a problem.
Thanks,
ImageAnalyst
.
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