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Re: [OM] Flare redux: Sun Stars?

Subject: Re: [OM] Flare redux: Sun Stars?
From: Joel Wilcox <jowilcox@xxxxxxxxxxxxxxxxxxx>
Date: Sat, 08 Dec 2001 16:03:17 -0600
Hello Thomas,

At 04:25 PM 12/8/2001 +0100, Thomas you wrote:
At 23:42 07.12.01, Joel Wilcox wrote:
So I guess these are my questions:

1. Is this kind of sun star a type of flare?

No. Flare is reflections inside a lens (or camera), the star effect is diffraction. In this case you obviously used a lens with 8 aperture blades, and the uneven diameter of your lens opening creates spikes like this. If your aperture was perfectly round there would also be diffraction, but it would be spread out evenly around the sun.

Thank you! One applied definition of diffraction I just read described it as the spreading of a wave (as of light) as it passes beyond an obstacle into the area beyond the obstacle which is not really "exposed" to the wave's motion. If this is correct, the obstacle in the case of a sun star is the lens diaphragm. We record on film the diffraction of light passing the "notches" of the diaphragm, as you explained.

The sun star diffraction seems almost by definition to occur only in photographs of light sources.

I've added another image with sun stars to the page I posted earlier:

http://soli.inav.net/~jdub/sunstar.html

The new image is the first one on the page. This is a sunset image in which there are two sets of sun stars created by the sun itself. I didn't take the photo with any notion that I would get sun stars. Rather I was hoping to catch the beams breaking through the clouds. I got a bit of that and a bonus with the sun stars, I think.

2. What effect might the lens coating have on the creation of sun stars?

None.


Two new questions:

1. Are the beams that I was *trying* to photograph (in the sunset shot just added at the link above) also a result of diffraction of any sort?

2. Since diffraction in the case of a sun star happens prior to the light's hitting the rear element of a lens, are you certain that the characteristics of the lens, or at least of the rear element, have no bearing on the photograph of the diffraction (i.e., the sun star)?

What I'm getting at in the second question is whether an inherently less sharp or flare-prone lens might affect the breadth or length of the pointing of the diffracted light in the sun star.

Thanks also, John L., for your comments on this (past and I hope future).

Joel W.


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