Photographing Star Points
The Rule of 500 (or 600...)
To get night shots with star points, rather than smeared stars, you need to choose a shutter speed that won't show star movement.
The longer the focal length, the less time it takes for star movement to show.
Traditionally, the Rule of 600 is applied. This states that when using 35mm film or a full frame digital SLR, then divide 600 by the focal length of the lens. This is the longest shutter speed you can use before movement is seen. For example, using a 24mm lens on a full-frame camera:
600/24 = 25 seconds
But if you're going to make large prints, you're probably better off using the Rule of 500 instead.
500/24 = 21 seconds
If you're using a DX sensor then you need to factor that in;
Rule of 500 for "cropped" sensors:
500/(focal length x crop factor)
So a 24mm lens on a 1.5 cropped sensor (Nikon D300 for instance):
|Rule of 500||Rule of 600|
|500/(24 x 1.5) = 14 seconds||600/(24 x 1.5)= 17 seconds|
I've created a handy sheet containing both the Rule of 500 and Rule of 600 for full frame, 1.5, and 1.6 crops. You can download the pdf HERE.