How to work out your night sky exposure

The earth turns, so stars trail. How long you can expose before they do depends on more than the old rule suggests.

Stars move across the frame continuously. The question is how long you can leave the shutter open before that movement shows as a streak rather than a point.

Step-by-step

1. Enter your sensor and focal length.
2. Compare the NPF and 500 rule results.
3. Plan the total integration time if you are stacking frames.

The 500 rule and why it is optimistic

Divide 500 by the focal length for the maximum seconds. Simple, memorable, and derived when people judged sharpness from prints rather than by pixel-peeping a 45-megapixel file.

It ignores pixel density entirely. On a modern high-resolution sensor, trailing is visible well before the 500 rule's limit, because each pixel covers a smaller piece of sky.

The NPF rule

The NPF rule accounts for aperture, pixel pitch and declination, so it gives a shorter and more realistic figure. On a dense sensor it may suggest half what the 500 rule does.

Use NPF when you want genuinely round stars at full size. The 500 rule remains fine for images shown at web sizes, where the trailing simply does not resolve.

Stacking beats a single long frame

Rather than one exposure at the trailing limit, take many at that limit and stack them. Noise is random and averages out; the signal does not. Twenty thirty-second frames give a far cleaner result than one ten-minute frame, and no star trails.

Practical settings

• Widest aperture your lens has, perhaps stopped down a third if coma is bad at the corners.
• ISO 1600 to 6400 on most modern cameras.
• Manual focus on a bright star using magnified live view. Infinity marks are not reliable.
• Shoot raw. The processing latitude matters more here than anywhere else.

Frequently asked questions

Which rule should I use?

NPF if the image will be viewed large or at full resolution, since it accounts for pixel pitch. The 500 rule is adequate for web-sized output.

Why does my modern camera trail sooner than the 500 rule says?

Because it has smaller pixels. The same angular movement covers more pixels, so trailing becomes visible at shorter exposures than the rule assumes.

Is it better to take one long exposure or several short ones?

Several short ones, stacked. Noise averages out across frames while the signal accumulates, and each frame stays under the trailing limit.