Usually referred to as the rule of 500 or the rule of 600 – saying that you take the focal length of the lens and divide that by 500 (or 600) to work out the shutter speed that you should be shooting at to avoid star trails.
I’ve seen the formula broken up into something a bit more mathematic detail based on factors such as how large do you want to print image. So using the rule of 500 and shooting at 16mm on a full frame camera – then I should shoot at say 31s.
Generally speaking – I find that lens faults are more of a concern for image quality is my glass. I’m shooting a lot of the time with a Tokina 11-16 (love the lens by the way). However the issue with a lot of wide angle lenses is that it stretches the edges of the frame. This is one of the reasons why I shoot panoramas with a lot of overlay, because centre sharpness is utilised more.
Generally – I feel that a shorter shutter is better since it means that there is less trailing. However a deciding factor between 20-30+s is actually the foreground interest. How much do I need to expose the foreground so that I can see the details properly. If the foreground is dark, then I may expose longer in order to bring out more detail and if it’s light, I may expose shorter. This is even more pertinent if using flash as a lighting source. Remembering that you expose for the ambient and then use the flash to shape and focus.
Convergence – was taken at 37s exposures as the foreground was fairly dark. Note that nowadays I’d also have put up the ISO.
The Dying Earth was taken using 25s exposures. The salt lake was very reflective. Noting though that I was also shooting at a higher ISO. Exposing a bit more allows for more flexibity in post processing to reduce noise.
Updating this post 15 September 2015
I’ve got a Tamron 15-30mm lens have have done some additional tests. Shooting at 15mm on a full frame – if you applied 500/15 = 33.33s. I’ve done a series of shots at 30s, 20s and 10s – you can see that if you zoom in close enough with 30s, trailing is quite evident. Whereas not so noticeable at 10s. But zoomed out, it’s not really noticeable. Emphasising here that it depends how big you want to enlarge the image.