Cursed wrote:Considering how far away the moon is, wouldn't you just be focussing at infinity? (I know nothing about astrophotography, by the way).
Yes. But that's usually a tiny bit back on the lens... It will focus past infinity. Usually that's the case for zoom lenses... Some manual focus lenses will have a hard stop right on infinity. I've often pondered why, but never really looked into it. There could be a few reasons... AF motors might not like constantly hitting a wall, and may slightly front of back focus. Shooting outside the visible spectrum also shifts focus due to different wavelengths of light, perhaps that's a reason.
Anyway, it can make it difficult to get focus critical. You certainly can't just stick it on infinity on the lens and assume it's in focus. What Kev said as well... use shutter release/timer. The mirror will also wobble a long lens... learn how to shoot with mirror lockup or use live view. Keep in mind live view heats up the sensor though... this creates additional sensor noise.