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.

> One has "correct exposure" when the film has the
> desired densities for certain object(s)/area(s).

Yes, but keep in mind that film also has a very high dynamic range (for film
in the dark regions) compared to a digital display.  I.e. the image linked
above was edited to correct for the bad display dynamic range to make the
dark areas visible approximately as they are to the human eye.

> Many automatic exposure systems tries to achieve a
> certain density on the film for objects with
> "medium tones".

I think this is what most people come to expect nowadays due to point and
shot cameras.  It is only natural to expect the same for computer generated
images.

    Thorsten

* About half the scanner resolution - the image was scaled down before.
Film resolution for the film that was used is 135 lines per millimeter
according to manufacturer at 1000:1 contrast ratio.

____________________________________________________
Thorsten Froehlich, Duisburg, Germany
e-mail: tho### [at] trfde

Visit POV-Ray on the web: http://mac.povray.org

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