# Star effect with lenses - why? how?

Started Sep 28, 2005 | Discussions thread
Re: Star effect with lenses - why? how?

David Iliff wrote:

I was wondering if anyone could explain exactly how the star effect
is formed.

A lens focuses light from a circular aperture into a point spread function. A point spread function IS the Fourier transform of a circular aperture. So, in effect, the process of focusing light is to take the Fourier transform of the incident wave function of light with respect to the linear aperture.

I was I was wondering if anyone could explain exactly
how the star effect is formed.

The star effect (spikes) is what happens when the aperture is noncircular, or when the aperture is partially opaqued (like the spider vanes in reflecting telescopes).

When the aperture is opaqued accross the aperture, the Fourier transform creates spikes at +90 degrees to the aperture blade and at -90 degrees to the aperture blade. When an aperture is opaqued at the edge, the fourier transform creataes a single spike rather reminisent of 'flare' from one side of a point-like object.

When there are an odd number of blades evenly spaced around the aperture, there are twice as many spikes as there are blades. When there are an even number of blades around an aperture there are still twice as many spikes, but pairs of them line up, so you see the number of spikes equals the number of blades.
--
Mitch

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