Essential Settings for Milky Way Photography: The 500 Rule, Aperture and ISO

Milky Way photography works the opposite way to daytime photography: your only constraint isn’t light, it’s the rotation of the Earth. If you keep the shutter open longer than needed, the stars stop being points and leave trails. In this article we cover the 500 rule that helps you find that limit, and the other three settings.

First, calculate the shutter with the 500 rule

If you’re shooting on a tripod without tracking hardware, the 500 rule gives a quick starting point: 500 ÷ focal length = maximum shutter (seconds). For example, with a 24mm lens 500 ÷ 24 ≈ 21 seconds, and at 14mm 500 ÷ 14 ≈ 35 seconds. Exceed this and the stars begin to turn from points into streaks.

For a more precise result, the NPF rule, which also accounts for sensor size and pixel density, is used; since it’s tedious to calculate by hand, it’s practical to check it via apps like PhotoPills. For a quick estimate in the field, the 500 rule is usually enough.

Open the aperture as wide as possible

In night-sky photography the goal is to gather as much light as possible. Use the widest aperture your lens allows (f/1.4–f/2.8). At a value narrower than f/2.8, the faint detail in the sky and the texture of the Milky Way disappear.

Start ISO in the 3200–6400 range

The typical ISO range for Milky Way shots is 800–6400; it varies with light pollution, moonlight and your camera’s sensor performance. Away from the city, under a dark sky, ISO 3200–6400 is a good starting point. If your camera’s noise performance is poor, stopping ISO down a little and lengthening the exposure (while still obeying the shutter rule) can give a cleaner result.

Gear: a sturdy tripod and a remote release

In night photography even the slightest vibration creates noticeable blur in a 10–30 second exposure. A sturdy tripod, a remote release (or a 2-second timer) and, if possible, a wide-angle, fast-aperture lens make up the basic gear list.

Focusing: focus on a star, not on infinity

Autofocus almost never works in night conditions. Set the lens to manual, zoom in on the brightest star or a distant light in Live View, and turn the focus ring until that point looks smallest and sharpest. Don’t trust the “∞” mark on the lens — on most modern lenses this mark can be a few degrees off from true infinity focus.

Common mistakes

• Pushing the shutter far past the 500 rule: the stars come out as short streaks instead of points.
• Leaving the aperture at a narrow value like f/5.6: not enough light is gathered, and the sky looks empty and noisy.
• Trusting autofocus: in the dark the focus system usually can’t lock; manual focus is essential.
• Not accounting for moonlight: on full-moon nights the sky brightens and the Milky Way’s contrast drops greatly — shoot during the moonless or new-moon period.