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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