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Re: [OM] Flare redux: Sun Stars?

Subject: Re: [OM] Flare redux: Sun Stars?
From: Thomas Bryhn <thomas.bryhn@xxxxxxxxxx>
Date: Sun, 09 Dec 2001 13:55:36 +0100
At 23:03 08.12.01, you wrote:

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.

A popular mental picture of diffraction is that it is the bending of light passing near an obstacle. This is *NOT* technically correct, but it's still a useful picture if you're a photographer and not a mathematician. A little more technically, every light source is a source of waves and it's only by catching *all* the light that a lens can reproduce a perfect picture of the source itself. By using obstacles of different shapes you are in fact choosing what kind of information to include in the picture, and therefore also which imperfections to be stuck with in the image.
The mathematics behind this is definitely not suitable for ASCII communication.

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

The diffraction occurs everywhere in photography, every point your lens is imaging is smeared in the same way. It is however very weak, and it's only in these special circumstances (very bright sources on dark surroundings) that you'll be able to see it directly. Take a look at Gary Reese's lens tests, and you'll see that as lenses are stopped down their contrast will suffer. Guess why...

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.

This is interesting but not different from the previous examples. You have a cloud in front of the sun, effectively dividing the sun into two light sources. You'll see that both sources have their own "sun stars", or diffraction stars.

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?

Particles and pollutants in the air will spread light, and the beams you see are beams not stopped by the clouds further back. In Star Wars movies the (laser) beams are visible in empty space, but in real life there must be some kind of spreading going on for you to see a beam (unless it hits you directly in the eye). A laser pointer should easily prove that. Stricly speaking diffraction and spreading can't be treated seperately, as spreading in front of your lens is also restricting information, i.e. you aperture is not only the aperture blades, but everything stopping light, like dust in/on your lens.

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.

A bad lens will degrade images of everything, but a SC lens won't smear your "sun stars" more than it will smear your clouds, rocks, or whatever. Lens construction could possibly play a role, but that has to do with placement of the aperture, definitly not with coatings.
Keep shooting these great pictures, don't worry about the lenses.

Regards,
Thomas Bryhn

PS! Have you ever tried squinting at the sun?


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