The first question to ask about a lens is of course its field of view - how much of the sky does it capture?
Figure 7.1 and Table 7.1 give the field of view of several common lenses with DSLRs that have an APS-C-size sensor. The numbers in Table 7.1 are exact for the Canon Digital Rebel family (EOS 300D, 350D, 400D); with the Nikon D50, D70, and their kin, the field is about 5% larger.
Field of view is measured in degrees, and some lens catalogs give only the diagonal "angle of view" from corner to corner of the picture. The exact formula for field of view is given on p. 58.
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