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Regarding Franks question: "How do you explain the steadily increasing
sun diameter towards the sides of the picture?"

My guess is that the photos were at least partially made with
automatic exposure settings. With the sun at the edges of the frame
the centre was darker resulting in either in a greater aperture and/or
a longer exposure time and thus in an overexposed sun at the edge.

Regarding George's different comments on using the sun as reference
for measurements with digital photos. With Greg we followed this
subject further off list. It was found that it is important how the
camera-lens-system is calibrated. This means that the attainable
precisions and accuracies of the measurements depend on how the photos
are made and measured afterwards, i.e. the camera-lens-system but also
the photo editing software, the technique used for measuring and the
personal skills. This suggests that the calibration and later the
measurements should preferably be done by the same person and using
the same techniques. It seemed to me that the precision one possibly
attains this way should be sufficient in order to use this technique
also in practice for CN. Greg may tell us one day more on this.

Marcel

   

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