What is optical vignetting, and why does stopping down reduce it?
Asked 12/8/2023
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I’m trying to understand optical vignetting in lenses. I’ve read that it is different from natural light falloff, and that it becomes stronger at wide apertures but improves when the lens is stopped down.
What exactly is happening geometrically for off-axis image points? How does the lens barrel or other parts of the lens block peripheral rays, and how does that change the apparent shape of the entrance pupil near the corners (the “cat’s-eye” effect seen in bokeh)? Also, is optical vignetting related to coma, or are those separate issues?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
2y ago
2 Answers
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I don't fully understand all of the questions, but I'll answer what I can.
Rays from off center do bend; the issue with the drawing is that it is including only selected rays. I.e. all points on the objective lens receives light from all points within the scene; and bends those rays to focus in the correct location. I added another ray to show this.
The entrance pupil is the size of the aperture as magnified by the objective element. As such, it cannot be larger than the element itself, but the objective element can be as large, or even larger than the lens barrel. When the size of the entrance pupil is equal to the size of the lens barrel you get vignetting because the images at the periphery include the edge of the lens barrel (hood/etc) within their field of view.
I.e. all points on an objective element includes light from all parts of the scene required to make a complete image as scene from their perspective. I.e. larger apertures stacks an increasing number of virtual images. And apertures that include the edge of the objective element are more likely to include the lens barrel within their field of view.
Originally by user70370. Source · Licensed CC BY-SA 4.0
user70370
2y ago
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Optical vignetting is mainly mechanical shading of off-axis light rays by the lens barrel or other lens parts. For image points near the center, the entrance pupil appears roughly circular. For points near the frame edges, part of that pupil is obscured, so it appears clipped or elliptical—often seen as “cat’s-eye” bokeh highlights.
At wide apertures, the entrance pupil is large, so off-axis points are more likely to “see” the edges of the barrel and lose some rays. That reduces illumination in the corners. When you stop down, the aperture itself becomes the limiting opening in the middle of the lens, and that smaller opening is more fully visible even from off-axis positions. So less of the ray bundle is blocked, and optical vignetting decreases.
This is separate from natural illumination falloff (often described by the cos^4 law), which also darkens the edges even without mechanical blockage. In real lenses, both effects can contribute.
Coma is a different aberration: it changes the shape of point images, especially off-axis, rather than simply reducing brightness by clipping the ray bundle.
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