In article
>Recently I started wondering about a property of lenses for which
>I cannot find an explanation myself. Upon photographing a scene with
>some small, bright light sources, such as street lights in night
>photography, it so happens that there are star-shaped extensions of
>the light source in the picture. Something like the effect of a
>star filter, but less dramatic of course. This property obviously leads
>to a loss of contrast, and the extent may vary from lens to lens.
>The part that I don't understand is how lens imperfections can
>lead to star-shaped distortions. As far as I know, all lenses are
>spherically symmetric for rotations around the optical axis, so there
>should be no reason for (scattered?) light to be more intense in some
>directions than in others. I recognize that in general the diaphragm
>is not spherically symmetric, but I cannot imagine this could have
>such an effect.
You have named the cause...
Since a point light source photographed against a dark background is an
ideal way to photograph the effects of diffraction, you can clearly
see those effects. Aperture selection and diaphragm shape will alter the
amount and character of the effect, but the effect is inherent in optics
and does not change with different lenses. BTW, this effect is also clearly visible when photographing bright sun reflections on water at small stops. Makes one wonder about how wonderful a lens could be if optimised at maybe f2.8-4, removing much of that diffraction-caused point spread for each
subject point rendered by the lens...
Hope This Helps