Non-astronomers seeing an astronomical photograph often ask what magnification or "power" it was taken at.
In astronomy, such a question almost has no answer. With microscopes, it does. If you photograph a 1-mm-long insect and make its image 100 mm long on the print, then clearly, the magnification of the picture is x 100.
But when you tell people that your picture of the Moon is 1/35000000 the size of the real Moon, somehow that's not what they wanted to hear. Usually, what they mean is, "How does the image in the picture compare to what I would see in the sky with my eyes, with no magnification?" The exact answer depends, of course, on how close the picture is to the viewer's face.
But a rough answer can be computed as follows:
"Magnification" of picture = —
That is: If you looked through a telescope at this magnification, you'd see something like the picture. Here 45° is the size of the central part of the human visual field, the part we usually pay attention to, and is also the apparent field of a typical (not super-wide) eyepiece.
So a picture that spans the Moon (half a degree) has a "magnification" of 90. Deep-sky piggyback images often have rather low "magnification" computed this way, anywhere from 10 down to 2 or less.
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