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    Re: Which diameter of the sun in digital photos ?
    From: Greg Rudzinski
    Date: 2009 Aug 21, 11:26 -0700

    Marcel,
    
    I agree with George's comments and would like to add that my Canon
    DSLR Rebel camera produces reliable images when way under exposed in
    black and white. Try using no filtering with settings of f22, 1/4000,
    ISO 100 and then increase the amount of light incrementally until the
    horizon is just visible as a reference. Also try a pair of opposing
    circular polarizers held out in front of the camera lens and adjusted
    to produce a faint image of the sun with a bright horizon beneath it.
    Experiment to find the best combinations for your particular camera
    and lenses.
    
    Greg
    
    On Aug 21, 4:49�am, Marcel Tschudin 
    wrote:
    > For investigating the refraction near the horizon I perform systematic
    > observations by photographing sunsets. So far I used the horizontal
    > diameter of the sun as a reference scale (pixel per degree) for
    > measuring the sun's position above the apparent horizon.
    >
    > Recently I compared this scale to one derived from pictures of
    > landscapes and found to my surprise that they differ by around 8%; the
    > sun is systematically slightly larger.
    >
    > The scale in landscape photos has been measured with distinct features
    > and the distances measured with Google Earth. The scale for (not
    > overexposed) sun photos has been calculated with the angular size of
    > the sun for the corresponding days; this size is based on the sun's
    > diameter (of the photosphere) and the earth's distance.
    >
    > The difference is not related to refraction since the measurements are
    > made in horizontal direction. �Any idea, why those two scales differ?
    > It is known that some CCDs have a leak in the IR, also the one in my
    > camera. Could this be responsible for it? Does anyone know more on
    > this?
    >
    > Marcel
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