But wait a minute - instead of a lens, should you be looking for a small telescope, perhaps an f /6 "short-tube" refractor? Some of these have ED glass or even three-element lenses and perform very well for astrophotography.
In my opinion and experience, good telephoto lenses perform even better. After all, they have more elements and more sophisticated optical designs. If two or three lens elements were enough, even with ED glass, that's how camera makers would build telephoto lenses. It isn't.
7.1 Why you need another lens
Figure 7.1. Field of view of various lenses with Canon or Nikon APS-C sensor compared to the Big Dipper (Plough).
The advantage of the telescope is its versatility. You can use it visually as well as photographically, and it may well come with a mount and drive of its own, whereas the telephoto lens would have to be piggybacked on an existing telescope. It may also accommodate a focal reducer, which telephoto lenses never do. There is certainly a place for both types of instruments in astrophotography.
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