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    Re: Which diameter of the sun in digital photos ?
    From: Marcel Tschudin
    Date: 2009 Aug 21, 21:19 +0300

    George, you wrote:
    
    > A discrepancy of 8% is hard to accept. Something is wrong, as Marcel
    > suspects. And such a big discrepancy should be easy to check for.
    
    Yes, the discrepancy is big. I thought I would have a reliable scale
    from my sunset observations. This scale is now put in question. It
    turns out to be not so easy, that's why I start to ask around, hoping
    that someone may have done a similar experience and may have even
    found the reason for it.
    
    > I suspect the angular calibration of his camera Perhaps he should check that
    > more simply, in terms of the angle subtended by an object a few metres away,
    > rather than by relying on distant topography. Near enough to that he can
    > check dimensions with a tape, yet large enough so that the camera focus
    > doesn't need to change from infinity, and also so that uncertainty in the
    > position of the effective optical centre of the lens is unimportant (within
    > much better than 8%).
    
    One reason could be distortion, but I wouldn't attribute it to barrel
    or pincushion distortion for the following reason: These optical
    distortions are important when referring to all of the image. The 6x
    zoomed pictures of the sun or the geographical feature (an island with
    a well defined shape) use only a very small part of the full picture
    size and they are usually near the centre. With the resolution used
    for those images the full size of the pictures is 958 x 1278pixels
    compared to the size of the island about 60 x 210pixels or the
    diameter of the sun about 105pixels. Within this small range these
    distortions are likely to be negligible.
    
    It doesn't seem to be a distortion of the computer screen when
    measuring the size of the geographical feature with GoogleMap. In
    order to verify this I made the measurement also with GoogleEarth
    which has a tool for measuring distances; the result was the same.
    
    > Alternatively, even though Marcel considers that the Sun image is not
    > overexposed, perhaps it is, and he is looking at a bright halo of light
    > around it.
    
    The scale derived from the sun is a statistical result from over
    hundred measurements, having a standard deviation of a little bit less
    than +/-1.5% or pixels. Indeed, I do also have overexposed photos, but
    they don't enter this statistics. The colour of the sun's
    circumference is a good indicator.
    
    Any suggestions for possible reasons are welcome! In the meantime I
    try to get a better estimation for this difference.
    
    Marcel
    
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