Why do out-of-focus highlights turn into partial circles near the edge of the frame?
Asked 12/22/2017
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When shooting wide open with shallow depth of field, I sometimes get round bokeh highlights, but near the left or right edge of the frame they become clipped or look like a "crescent"/"cat's eye." Is this caused by the lens barrel blocking part of the aperture, or something else? Why does it usually get stronger toward the frame edges, and why might it occasionally look uneven?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
2 Answers
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It's often called "cat's eye" bokeh. Most often it is seen in fairly wide angle lenses with very wide apertures.
"Cat's eye" bokeh is the result of using a very wide aperture lens where the entire entrance pupil is not visible from the angle of the widest light rays in the field of view. This results in the bokeh being vignetted into an irregular shape. The closer to the edge of the field of view the light source is, the more pronounced the distortion of the bokeh shape will be. Highlights coming from light sources near the center of the FoV will be round or nearly round because the entire entrance pupil is visible on the front of the lens from their position. Stopped down the same lens will not demonstrate the effect even for the edge rays. If a lens is designed with uncorrected field curvature, spherical aberration, or astigmatism this can make the cat's eye effect even more pronounced and/or swirly.
When such a lens is stopped down, the full area of the lens' entrance pupil (the aperture as viewed through the front of the lens) can be seen from the field of view covered by the lens.
In the case of your example photos it is not only out of focus highlights that are demonstrating a "cat's eye" effect. It is also lens flare due to reflections inside your lens. The same principle is at work, though. The reflections aren't all circular because they're being shaped by the edges of the things they are passing through and the shapes of the edges of the things off which they are reflecting .
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
8y ago
0
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This is the normal cat’s-eye bokeh effect. Near the center of the frame, an out-of-focus point can “see” the full entrance pupil of the lens, so the blur disk looks round. Toward the edges, part of that pupil is no longer visible from that angle, so the blur disk gets vignetted and turns into a clipped oval or crescent shape.
So yes: in practical terms it’s related to the lens optics/aperture being partially obscured for off-axis rays, not a sensor issue. The effect is strongest when:
- shooting wide open
- using a very fast lens
- the highlight is near the edge of the frame
If you stop the lens down, the effect is usually reduced because the effective pupil is smaller and more fully visible across the frame.
If some highlights look a bit irregular, lens design factors can contribute as well, including things like field curvature or other off-axis aberrations. But the main explanation for your examples is standard cat’s-eye bokeh caused by mechanical/optical vignetting of out-of-focus highlights near the frame edges.
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