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What steps could someone take in order to keep their bokeh balls as circular as possible?

I know stepping down the lens aperture will help. Are there any other suggestions?

Would using the lens hood help?

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  • Use a lens with a narrow enough field of view that the entire entrance pupil is visible when viewing the front of the lens from the anywhere within the field of view captured by the camera. Lenses that demonstrate cats eye bokeh are typically wider angle lenses with the optical elements recessed from the rim of the lens barrel.

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  • Use a lens sufficiently corrected for astigmatism and field curvature.

For more about how these things contribute to cat's eye bokeh, please see:
What is the cause of this non-uniform bokeh effect?
How can I take pictures with extreme bokeh with an in-focus subject nearby?
How do you achieve sharp bokeh circles?
What exactly is this light artifact/flare?

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  • Vignetting is typically the largest contributor to "cats-eye" bokeh. Your FoV point is very strange - you can vignette with an arbitrarily small FoV. – Brandon Dube May 3 '17 at 23:25
  • Very few lenses (if any) with narrow FoV demonstrate mechanical vignetting in the way that many wider angle lenses with front elements set well back into the lens barrel, at least when set to some focus positions, do. – Michael C May 4 '17 at 0:16
  • Please clarify what you mean by "mechanical vignetting" - vignetting in lens design is typically implemented by reducing the diameter of a lens element. – Brandon Dube May 5 '17 at 0:09
  • @BrandonDube The lower left lens in the illustration is an example of mechanical vignetting. It's not typical because the vignetting is not uniformly circular. – Michael C Nov 28 '17 at 18:17
  • It's perfectly typical. Vignetting can't be "uniformly circular" - that would require that moving in +y caused some new limiting aperture to be introduced in +y, -y, +x, -x. This is unphysical. Further, it is immaterial whether a ray was clipped by a mechanical surface or an optical one. – Brandon Dube Nov 29 '17 at 0:59
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I pretty much think stopping down is your only path of recourse or simply using a slower lens. :) I never ran into catseye bokeh with an f/1.8 lens. I only ever saw it with an f/1.2 used wide open.

A lens hood sounds like it might work, depending on the angles involved. For me, the claim that catseye only happens at the edges/corners of the frame isn't necessarily 100% true...

example of catseye in center of frame

So, it does depend on where your light sources are, and what their angle to your lens is, and whether or not the hood can shade out the ones that are hitting at more oblique angles.

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