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The high resolution of
digital sensors provide quite a challenge for normal
photographic lenses. These lenses were made for general daytime
photography. Shooting pinpoint light sources across a wide field
if stars is the most difficult test possible for a camera lens.
Telescopes are corrected to give their best performance at
relatively slow focal ratios at infinity. Camera lenses must
cover comparatively large fields at various subject distances at
much faster focal ratios. So it is not really fair to compare
the performance of a camera lens to an astronomical telescope,
especially when the camera lens is used wide open.
There are two type lenses:
fixed-focal length lenses and zoom lenses. The zoom lenses
usually perform ok for daytime work, but do not
perform as well as fixed-focal length lenses, especially for
astrophotography. They contain more elements in more complicated
optical designs, and some of them are usually slower in terms of
their focal ratios.
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