Can someone explain lens cast by aperture to me

Started 4 months ago | Discussions thread
Alan WF
Alan WF Senior Member • Posts: 2,896
Re: Can someone explain lens cast by aperture to me

I’m not sure what you’re saying here. Yes, light at different wavelengths takes different paths through optics and has different PSFs in the final focal plane. But which of these impact on color?

On one hand, I can see chromatic aberration causing wavelength-dependent vignetting. For example, the image of the iris can suffer chromatic aberration as seen from the objective; it can pass more or less light at different wavelengths and I can see this causing a color shift. (I’d note that this should be easy to see just by looking at the iris through the objective.)

On the other, if the PSF at one wavelength is more fuzzy at wider apertures, I can again see this causing a color shift.

However, there is a difference, I think.

Consider a lens that gives a fuzzier blue image than a red image (since in my experience chromatic aberration is more of a problem in the blue) and moreover a more fuzzy blue image at wider apertures. This would give a color shift for small features, but not for large features. That is, a grey card would not show a color shift.

Conversely, if the lenses suffers wavelength-dependent vignetting at the iris, I’d expect the color shift to be independent of the spatial scale of the feature.

Any idea which is it?



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