A good rule of thumb is (focal length / 500) to figure out your maximum exposure time before having blurry stars. To get around a 20-30s exposure you're lookin

A good rule of thumb is (focal length / 500) to figure out your maximum exposure time before having blurry stars. To get around a 20-30s exposure you're looking at somewhere between 14-24mm focal length, ideally with the largest aperture you can afford.