On 27 Aug 1998 22:18:59 +0200, Steinar Midtskogen wrote:

>Compare, if you like, the two photos on:
>
>
>
>They're both taken with a Nikkor 200mm (& FM2), the first at f/4 (wide
>open), the second at f/32. The streetlights on the f/4 photo have
>soft glows, whereas those on the f/32 photo have 18 spikes evenly
>distributed around the light centers. I did the same test with my
>Nikkor 50mm too, at f/1.4, f/5.6 and f/11. There are soft glows on
>the f/1.4 one, 14 tiny spikes on the f/5.6 and the same, but sharper,
>on the f/11 one. I'm not sure whether I like the effect or not.
>
>I compared the number of spikes with the number of blades of the
>diaphragm in each lens, but I found no correspondence.
>
>Can anyone explain the phenomenon?

It is caused by diffraction around the diaphragm blades (which is why
the effect is stronger at smaller stops), which with an odd number of
blades should give twice the number of points in the image as the
number of blades (7 and 9...;-). Does it?