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Re: (meteorobs) Re: meteorobs-digest V3 #264
On Tue, 11 Jan 2000, T. M. wrote:
> Slightly off topic, but...
Not for telescopic.
> 1) In images - is "field of view" measured across the
> diagonal of the image - or is it typically only specified for the
> shortest dimension of the image?
For telescopic it's circular, so orientation doesn't matter. With the
unaided eye you can see more laterally than vertically. The
oft-quoted 100 degrees field of view is an approximation as it's more
of an ellipse, and seems to correspond to the shorter dimension. For
photography, people should quote both dimensions or a scale (say
degrees per cm), but often the larger angle is quoted for commercial
reasons.
> 2) what, if any, is the mathematical relationship between magnification of a
> lens
> (of a telescope or camera) and 'field of view' (degrees)?
For a telescope it depends on the eyepiece. The true field of view is
the apparent field of view of the eyepiece divided by the
magnification. The apparent field is the angle subtended by the edge
of the field at your eye. If you hold the eyepiece to the sky or at
the telescope with your other eye open, you can estimate the angle.
Eyepiece apparent fov varies from about 20 degrees for the solid
Tolles and Monocentric to about 90 degrees for some designs. So there
is no simple answer.
For a camera, it depends on the film format. For a 35-mm camera its
about 50 degrees for a 50-mm lens. The fov goes inversely with the
focal length. So with a 100-mm focal length lens the fov is about 25
degrees. The magnification is proportional to the focal length.
Malcolm
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