Archived posting to the Leica Users Group, 2001/02/01

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Subject: [Leica] Re: Star Patterns
From: "Henning J. Wulff" <henningw@archiphoto.com>
Date: Thu, 1 Feb 2001 14:29:46 -0800
References: <sa796c8f.008@smtp.usip.com>

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.

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    *            Henning J. Wulff
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