Archived posting to the Leica Users Group, 2001/02/01
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]
At 2:02 PM -0800 2/1/01, Robert Rose wrote:
>I have a different view on the "star pattern" phenomena. I have
>found it is related to the number of blades, but not how you might
>think. It is related to the intersection points; the discontinuity
>where the blades overlap.
>
>I have the slides to show this from a study I did.
>
>A lens, such as a Hektor, with lots of blades, will not exhibit this
>at apertures which form almost a perfect circle.
>
>Stopped down, a lens is more likely to have such sharp
>discontinuities between blades. It is also necessary that the light
>be sharply in focus. At stopped down apertures, it is more likely
>that the light source is in sharp focus.
>
>One lens, the 35-70 f/4, will exhibit this at wide open aperture.
>Examining the blades shows that the discontinuities appear on that
>lens when wide open.
>
>By the way, a poor quality lens will not exhibit this star pattern.
>It is a signature of quality. Flare in a lens will also mask the
>effect.
>
>Robert Rose
Sorry, I wasn't clear in the previous post. The edges have to be
straight, or fairly close to straight for this to work. It's not the
intersections that cause diffraction.
Yes, the light source has to be fairly close to a point source for
the whole thing to work, both as source and as image, which implies
in-focus. In optical demonstrations, point sources are used to show
diffraction effects.
- --
* Henning J. Wulff
/|\ Wulff Photography & Design
/###\ mailto:henningw@archiphoto.com
|[ ]| http://www.archiphoto.com