How to shoot the night sky
Stars are simple subjects with a brutal constraint: Earth never stops moving. Get the shutter speed right and you capture pinpoint stars. Get it wrong and every star becomes a trail. Everything else — ISO, aperture, focus, location — flows from solving that one problem first.
The core problem: Earth's rotation
During a long exposure the Earth rotates under the stars. From the camera's perspective, the stars drift across the frame. Leave the shutter open long enough and that drift becomes visible as streaks. How long is "long enough" depends on two things: your focal length and your camera's sensor size. A longer focal length magnifies the drift. A smaller (crop) sensor uses a narrower angle of view, which has the same magnifying effect on apparent star motion.
This means there is a hard ceiling on your shutter speed — push past it and you lose the pinpoint stars that make a Milky Way shot work. Every decision in night-sky photography is essentially about staying under that ceiling while gathering enough light to get a usable image.
The 500 rule
The long-standing rule of thumb is simple:
max shutter (seconds) = 500 ÷ (focal length × crop factor)
For a full-frame camera (crop factor 1.0) shooting at 24 mm, that gives roughly 20 seconds. Shoot an APS-C body (crop factor 1.5) at the same 24 mm focal length and your effective angle of view is narrower, so stars appear to drift faster — the formula says about 14 seconds. A Micro Four Thirds body (crop factor 2.0) at 14 mm comes out to about 17 seconds.
The crop factor matters because what controls star trail length in your final image is not the focal length stamped on the lens — it is the effective focal length relative to full frame. A 24 mm lens on an APS-C body behaves like a 36 mm lens on full frame, so you apply the crop multiplier before dividing. Use the 500 rule calculator to find the exact limit for your camera and lens in seconds.
The 300 rule and why resolution matters
The 500 rule was developed when 12–18 megapixel sensors were typical. Modern sensors pack far more pixels into the same sensor area, so each pixel covers a smaller angle of sky. That means the same amount of star drift occupies more pixels — and becomes visible sooner.
A conservative starting point for high-resolution bodies (24 megapixels and above on full frame, or most current APS-C cameras) is the 300 rule: simply replace 500 with 300 in the formula. At 24 mm on a full-frame body that cuts you from 20 seconds to 12 seconds. It is not as catchy, but it is more realistic for the cameras most people are shooting today.
The practical takeaway: if you have a high-resolution body and you are seeing trails in your test shots even within the 500-rule limit, drop to 300. The 500 rule remains a useful ceiling to start from; the 300 rule is where many shooters end up in practice.
The NPF rule: a more precise modern alternative
Both the 500 and 300 rules are single-variable shortcuts that ignore a lot of the physics. The NPF rule (named for the variables it factors in: aperture, pixel pitch, and declination) takes a more complete approach.
The key insight is that pixel pitch — how large each individual photosite is, in microns — sets how much drift it takes to smear a star across multiple pixels. A sensor with large photosites can tolerate longer exposures than a densely packed one at the same focal length, even if both sensors are the same physical size. Aperture also plays a role: a wider aperture produces a slightly larger point spread for each star, which gives you a small amount of extra tolerance. And the declination (how far a star sits from the celestial equator) matters because stars near the poles appear to move more slowly than stars near the equator — so you can push exposures slightly longer when your composition is aimed toward the pole.
You do not need to work through the full NPF formula by hand. The conceptual value is understanding why the answer differs between bodies: a 24-megapixel full-frame camera and a 45-megapixel full-frame camera at the same focal length have genuinely different maximum shutter speeds because their pixel pitches differ. The 500 rule calculator covers the common-case rules; the NPF rule is worth knowing when you are optimizing for a specific body and want to squeeze every second out of a single exposure.
Practical settings to start from
Walk through each variable and why it is set that way:
- Aperture: as wide as practical. f/2.8 lets in four times more light than f/5.6. Night sky shooters gravitate toward wide, fast lenses — 14 mm f/2.8 or 24 mm f/1.8 are popular choices. Very fast lenses (f/1.4, f/1.8) can show coma (seagull-shaped stars at the corners) wide open, so many shooters stop down one-third to one-half stop to clean up the corners. It is a trade-off worth testing on your specific lens.
- ISO: high, but not reckless. Your sensor's native ISO (often around 1600–3200) is the cleanest starting point. Modern sensors handle ISO 6400 with RAW processing and noise reduction far better than the same number looked five years ago. Shoot test frames and examine them at 100%; you will find the limit faster than any rule of thumb can tell you.
- Shutter speed: from the rule for your camera. Set the calculated limit from the 500 (or 300) rule calculator, shoot a frame, zoom in to 100% on a star in the corner, and assess. Adjust down if you see trails.
- Focus: manual, on a bright star or infinity. Autofocus fails in the dark. Switch to manual, zoom your live view to 10x on the brightest star visible, and turn the focus ring until the star collapses to its smallest point. Many lenses' infinity mark is not exactly at the infinity stop — confirm optically before you shoot a full sequence. The hyperfocal distance calculator is useful when you want both a sharp foreground element and sharp stars: focus at the hyperfocal distance and everything from half that distance to infinity is acceptably sharp.
- Format: RAW always. Night shots require significant post-processing — noise reduction, highlight recovery on the Milky Way core, shadow lifting in the foreground. JPEG throws away the data you need. Shoot RAW.
A quick exposure sanity check: use the exposure value calculator to confirm your ISO, aperture, and shutter combination lands in a reasonable EV range for the ambient conditions. On a moonless night at a dark site you are typically working around EV −6 to EV −4 — very low light values that feel counterintuitive if you are used to daytime photography.
Light pollution and planning
The Milky Way core is faint. Suburban skyglow can wash it out entirely. Two decisions make the biggest difference before you ever leave the house:
- Time your shoot around the new moon. A full moon is roughly as bright as a streetlit sky and will overpower the Milky Way. The window around new moon — a few days either side — is the reliable target. Apps like PhotoPills, Stellarium, and Planit show moon phase and rise/set times alongside Milky Way position.
- Find a dark site. The Bortle scale runs from 1 (pristine dark) to 9 (inner-city skyglow). The Milky Way core becomes photogenic around Bortle 4 or better. Light Pollution Map (lightpollutionmap.info) and the Dark Sky Finder show sky quality by location. Driving an hour out of a city often drops you two to three Bortle classes.
Once you are on location, give your eyes 20–30 minutes to dark-adapt before judging what the sky looks like. Use a red flashlight to preserve your night vision during setup.
Star trackers: beating the shutter limit entirely
Every rule above exists because the camera is stationary while the sky moves. A star tracker (also called a sky tracker or equatorial mount) solves this at the source: it rotates the camera at exactly the rate the Earth spins, keeping the stars stationary relative to the sensor. With a tracker you can expose for two, three, or five minutes at ISO 800 with a stopped-down aperture — and get far more signal with far less noise than any single untracked exposure.
The trade-off is that while the stars stay sharp, any foreground in the frame will blur. The standard approach is to shoot a separate untracked foreground exposure and blend the two in post. It adds complexity, but for serious Milky Way work a tracker is the single upgrade that most dramatically improves image quality. Popular options like the iOptron SkyGuider Pro, Sky-Watcher Star Adventurer, and Move Shoot Move are designed to be portable enough for location work.
Foreground and composition
A sky full of stars without context can feel flat. Strong foreground elements — a silhouetted ridge, a lone tree, a cabin with a light in the window, a winding road — give the viewer's eye an anchor and a sense of scale. Wide lenses (14–24 mm on full frame) naturally include a lot of foreground, which is one reason they dominate night sky work.
Scout your location during daylight. Identify where the Milky Way will rise relative to your subject using a planning app, and time your arrival so the core is at a useful angle when darkness falls. The Milky Way core is most visible from the Northern Hemisphere roughly March through October, highest in the sky around midnight in late spring and early summer.
Foreground sharpness is a separate challenge from star sharpness — especially with a wide lens that might have close elements just a few feet away. That is where the hyperfocal distance calculator earns its keep: set your focus at the hyperfocal distance and everything from that point to infinity is within your depth of field, keeping both the rocks at your feet and the stars overhead acceptably sharp in the same frame.