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.

Now, step back…it shouldn’t matter, right? Why would the focal length affect whether you see streaks? Same camera, same sky, nothing changed except how zoomed in you are. Which means the more “magnified” the image when you take it, the shorter the duration has to be or you will SEE the streaks. The streaks are likely to be there almost no matter what, but with a 500mm lens, you’ll notice them at anything over a second.

Why do I like the article? Because it goes through the math of perception based on how it shows up on the light sensor:

What Can We Conclude?

• Streaking starts a LOT sooner than any rule you may have learned.
• The time it takes to streak depends on the inter-pixel* distance (sensor density / mm) and the focal length.
• How much streaking to allow depends on your aesthetic tolerances.
• You can not get more or brighter stars by exposing longer; starlight has already given up on one pixel* and moved on to the next in just a few seconds.
• The longer the focal length, the more impossible it becomes to prevent streaking.
• Gaps in your star trails may be unavoidable if the inter-shot delay (normally 1 second) is long enough to skip pixels*.

Source: 600 Rule? | StarCircleAcademy.com llc

Clearly presented, clearly argued. Nice.