Long exposure and the night sky

Pinpoint stars or star trails?

Because Earth rotates, the stars appear to move across the sky by about 15° an hour. In a short exposure they stay as points; in a long one each star draws an arc around the celestial pole, near Polaris in the northern hemisphere.

For pinpoint stars you keep the exposure short enough that the movement does not show, usually 5 to 25 s depending on lens and sensor. For trails you want the opposite: many minutes or hours of total exposure.

Long trails are usually made from many 20 to 30 s frames merged in software rather than one huge exposure. That keeps noise and heat down and lets you remove a passing plane from a single frame.

The 500 rule, and why it is rough

The classic shortcut is: maximum shutter time in seconds ≈ 500 ÷ (focal length × crop factor). With a 20 mm lens on full frame that gives 25 s; on APS-C (1.5×) about 17 s.

It came from the film era, when prints were small and grain hid slight trailing. On today’s high-resolution sensors viewed at 100%, 500-rule shots often show short streaks. Many photographers use 300 or 400 instead, which is just as approximate.

It also ignores aperture and where you point: stars near the celestial equator move fastest, stars near the pole barely move.

The NPF rule

The NPF rule adds pixel size and aperture. A common simplified form is: time ≈ (35 × f-number + 30 × pixel pitch in µm) ÷ focal length.

Example: 20 mm at f/2.8 on a full-frame 24-megapixel camera (pixel pitch about 6 µm) gives (98 + 180) ÷ 20 ≈ 14 s, noticeably shorter than the 25 s from the 500 rule. On a 45-megapixel body with about 4.4 µm pixels it drops to about 11 s.

It is still an estimate tuned for sharp-looking stars, not a physical limit. The calculator on this page gives both results, so you can compare and test with your own camera.

When and where: Milky Way season, new moon, dark skies

The bright core of the Milky Way lies towards Sagittarius. In the northern hemisphere it is visible from roughly February or March (before dawn) to October (after dusk), with summer the easiest; in the southern hemisphere the core rides higher and the season is similar (approximate, depends on latitude).

Moonlight washes out faint stars, so plan around the new moon, or a few days either side, or go out after a crescent moon has set.

Light pollution is the biggest enemy. The Bortle scale grades skies from 1 (excellent dark site, the Milky Way casts faint shadows) to 9 (inner city, only a few bright stars). Even moving from class 7 to class 4, often an hour’s drive from town, makes a dramatic difference. Light-pollution maps help you find darker places.

Settings and light painting

A good starting point: wide lens (14 to 24 mm), widest aperture around f/1.8 to f/2.8, ISO 1600 to 6400, shutter from the NPF rule, RAW, and a white balance around 3800 to 4500 K. Focus manually on a bright star using magnified live view; autofocus rarely works in the dark.

Light painting means adding light during the exposure, with a torch, LED panel or even a phone screen. A short, gentle sweep across a tree or rock can give foreground detail without a separate exposure. Keep it dim and warm; a little light goes a long way at ISO 3200.

Be considerate at dark sites: other photographers may be exposing, so use red light and warn people before switching on anything bright.

Long-exposure noise reduction and stacking

Long-exposure noise reduction (LENR) takes a second, black frame of the same length right after your shot and subtracts its hot pixels. It works, but it doubles the wait: a 30 s exposure takes a minute. Turn it off for star trail sequences, because it creates gaps between frames.

Sensors also warm up during long sessions, and warm sensors are noisier. On a summer night of 77 °F (25 °C) you will see more hot pixels than at 32 °F (0 °C).

Stacking is the more powerful option: shoot 10 to 30 frames of the same sky and let software align and average them. Random noise falls while stars stay, giving a cleaner image than any single frame. Dedicated tools and tracking mounts are the next step if you get hooked.