Re: Stupid question about magnification

Vlad, I don't think you and Chris disagree on the particulars, only on
how to express them. If we are going to ask rhetorical questions, we
might ask you what the magnification is of a 2000 mm focal length
objective. That can't be answered with a specific number unless one
assumes a rather arbitrary figure for the spacing between the eye and
the focal plane. I say arbitrary because many people can focus on a
real image closer than 250 mm. For instance, I can approach to within
100 mm of an image or object before I can't focus on it anymore.

The best characterization is to give the image scale at the focal plane,
in my opinion. For instance, for a 2000 mm focal length objective, the
image scale is approximately 35 mm per degree. Since the Moon (say) is
about half a degree across, this scale tells us that the image of the
Moon at the focal plane, 2000 mm behind the objective, is about 17.5 mm
across.

One can then determine the angular size of that image as viewed from
any desired distance behind the focal plane. If it is viewed from 100
mm, the 17.5 mm image would appear to span about 10 degrees, yielding
an effective magnification of 20x; if viewed from 250 mm, it spans
about 4 degrees, for a magnification of 8x; and so forth. To me, this
seems a clearer and more direct way of describing the objective.

--
Brian Tung <brian@xxxxxxx>
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.

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