There is a rule that photographers use to figure out how long of an exposure you can do on a camera before you’ll start to see star trails. This assumes, of course, that you’re not TRYING to get star trails. Instead, you want those lovely little pinpoint stars. The classic rule says you take a set number of 600 and divide it by the effective focal length of the camera lens. If you have a nifty 50 lens, that means you would be able to do about 12 seconds of exposure before streaking occurs.

Most astronomers feel that the 600 number is a little high. More like 500. So then you would say 10 seconds. There’s a small extra factor in there if it is a crop sensor instead of a full-frame, and so for most DSLRs, you have to divide further by 1.6. So the article attached calculates that down to 7.5 seconds.

Perfectly logical, simple math. But as the article points out, that’s not entirely true. The premise is solid though, if you think about it. Let’s say you used one of those big honking 500mm lenses and the rule 500, ignoring the crop sensor. It would say that if you’re going to point it at a small section of the sky, you are so “zoomed in”, that you’re going to see streaks for anything over a 1s exposure.… Read the rest