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My suspicions are raised by Marcel's question- "Have you ever tried with the
same lenses to compare the pixels per degree you obtained for the sun with
those of landscape features?". Pixels per degree is by no means constant
where a standard non-zoom lens is being used .

Take a perfect lens, such as a pinhole, which images a rectangular object,
such as a flat-fronted block of a building, with no distortion at all, as a
precisely rectangular image on the film. If you have a 35mm film with a 38mm
focal  distance, pinhole to film, then the full width, side to side, across
the long dimension of the film, corresponds to about 50 degrees.

Near the centre, this corresponds to 0.663 mm per degree. Near one side,
across the long dimension, 25 degrees away, that's increased to 0.803 mm per
degree. It's a necessary consequence of the projection on to a flat plane.
In a digital camera, all those dimensions are correspondingly reduced, but
the proportional change is exactly the same.

Those dimensions correspond, roughly,  to the widest angle of an ordinary
digital camera with a standard zoom lens at minimum zoom.

However, Marcel is using a camera at 6x zoom, so the available angle will be
correspondingly less, and the distortion less too. But even so, if it's to
be used for angular measurement, the scale factor in mm per degree, needs to
be checked at different angles from the centre. Also, the two objects being
compared should only fill a small part of the screen, and should both be
well centred.

Another question relates to that 6x zoom. Is Marcel comparing the Sun and
the landmarks on the same photo, so that we can be sure that the zoom level
is the same for both? Or are they taken at different times, in which case we
need to be convinced that setting the zoom to (presumably) its maximum of x6
is reproducibly the same on each occasion.

Next is the angle subtended between two landmark points. Marcel tells us the
spacing between them has been determined by Google. Does that correspond to
old-fashioned paper mapping? How precisely has his own observation post been
located, as that's a factor in the calculation?

It might be useful if we were provided with some of the relevant numbers.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
----- Original Message -----
From: "Marcel Tschudin" 
To: 
Sent: Friday, August 21, 2009 8:38 PM
Subject: [NavList 9589] Re: Which diameter of the sun in digital photos ?


|
| Greg, thank you for your suggestions.
|
| For this exercise I'm using always the same camera having always the
| same lenses.The sun's diameter is measured in a photo-program after
| removing all colours except green and in some cases even removing all
| but blue. I fear to put an other hand-held filter in front since I
| have to measure the distance to the horizon which still must be
| visible, since the main purpose is to measure the sun's distance to
| the horizon. I have so many only faintly visible sun pictures
| confirming the obtained scale that I'm fairly confident that the value
| corresponds to what the camera 'sees'.
|
| Have you ever tried with the same lenses to compare the pixels per
| degree you obtained for the sun with those of landscape features?
|
| Marcel
|
| |


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