Perhaps the biggest disappointment to any beginning astrophotographer is finding out how hard it is to focus the camera accurately. With DSLRs, the problem is even worse than with film SLRs because the viewfinder is smaller and dimmer, and also because standards are higher.
We want to focus DSLRs more precisely than film SLRs because we can. Unlike film, the DSLR sensor doesn't bend. Nor does light diffuse sideways in the image. It's easy to view the image tremendously magnified on the computer, so we are much less tolerant of focusing errors than we used to be (to our detriment) back in the film era.
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