Thorsten Froehlich wrote:

> In article <4133a71f$1@news.povray.org> , Tor Olav Kristensen 
> <tor### [at] TOBEREMOVEDhotmailcom>  wrote:
> 
> 
>>I suspect that the colour bleeding around the sun
>>in this photo is caused by inperfections in the
>>camera lens.
> 
> 
> Not necessarily  keep in mind that film also is not infinitely thin.  For
> color film the depth light has to penetrate to reach all three layers is
> about three times the films resolution.  And the light scatters in each
> layer as well.  This creates a natural blur effect at sharp light/dark
> transitions.  I.e. <http://mac.povray.org/rhein.jpg> is (a after two JPEG
> compressions) scan of a positive film and the resolution in the image is
> about 80 lines per millimeter*.  Looking at the two bell towers clearly
> shows a kind of blur. But, the sunlight entered at about a 30 degree angle,
> so even without refraction it did pass at least two pixels.  Add scattering
> and you get exactly the blur effect seen.
...

The sun covers about 0.5 degrees of our field of view:

http://www.google.com/search?q=2*atan%28radius+of+Sun%2FAstronomical+Unit%29+radians+in+degrees


A camera with 35mm film (36 mm x 24 mm) and a normal lens (50mm)
will have a field of view of about 40 degrees:

http://www.google.com/search?q=2*atan%2836%2F%282*50%29%29+radians+in+degrees


Now I assume that the CCD (or film) in the camera that shot the
picture that Warp gave a link to, covers the whole field of view
of the lens. I also assume that the image was shot with a 50 mm
lens and that the image has not been cropped.

http://www.students.tut.fi/~warp/photos/patikka2/35.jpg
(That image is 1136 pixels x 856 pixles.)

If these assumtions are correct, then the image has
1136 pixels / 40 degrees = 28 pixels/degree

This means that if there were no distortions in the atmosphere,
inside lens or inside the film, then the sun would cover about
28 pixels/degree * 0.5 degrees = 14 pixels.
(See http://home.online.no/~t-o-k/35_Sun.jpg)


The colour bleeding in the image extends somewhere between 77
pixels and 296 pixels from the center of the sun.
(See http://home.online.no/~t-o-k/35_Bleeding.jpg)

If we can agree that the colour bleeding in the image is more
than 200 pixels from the center of the sun, then it will follow
that the radius of the colour bleeding on the film is more than

200 pixels / 1136 pixels * 36 mm = 6.3 mm


I measured a piece of 35 mm film to be about 0.14 mm thick.

6.3 mm / 0.14 mm = 45.3

Now I cannot make myself believe that the internal scattering/
refraction/reflections in the layers of a such a film will
extend as far as 45 times the thickness of the film.

-- 
Tor Olav
http://subcube.net
http://subcube.com

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