Night sky photography: the moon, star trails and the 500 rule
The Milky Way needs a dark sky and a new moon. The moon and star trails do not. Here is how to photograph both, and why the 500 rule only tells part of the story.
A full moon rising over Port Phillip Bay looks enormous. It hangs orange above the water, every crater visible to the naked eye. Then you take the photograph and the screen shows a small white dot, blown out and featureless, above a black bay. Nothing about it resembles what you saw.
The moon and the stars are the two night subjects most people try first, and both fail in the same way: the camera is set for the wrong thing. Neither needs the dark skies and new moon that the Milky Way demands. The moon photographs well from a city balcony. Star trails can be made from an outer-suburban backyard. What they need is a different approach to exposure, and a little planning.
How to photograph the moon
The moon is a grey rock lit by direct sunlight. That single fact explains most failed moon photographs. The camera's meter sees a vast black sky with a small bright dot, averages the scene and brightens everything, so the moon burns out. The fix is to expose for the moon alone.
The starting point is the Looney 11 rule: for a full moon, set f/11 and a shutter speed of one over your ISO. At ISO 100 that is 1/100 sec; at ISO 400, 1/400 sec. It is the lunar version of the old sunny 16 rule, and it works because the moon is in full sunlight. A half moon or crescent is dimmer, especially near the shadow line, so give it one or two stops more. Check the histogram and the highlight warning, and adjust from there.
Focal length matters more than anything else. As a rough guide, the moon's image on the sensor is about its focal length divided by 110, in millimetres. That is small.
| Focal length | Moon diameter on sensor | Share of full-frame height |
|---|---|---|
| 50mm | About 0.5mm | About 2 per cent |
| 200mm | About 1.8mm | About 8 per cent |
| 400mm | About 3.6mm | About 15 per cent |
| 600mm | About 5.5mm | About 23 per cent |
On an APS-C or Micro Four Thirds camera the moon fills a larger share of the frame at the same focal length. Even so, a detailed moon on its own needs 400mm or more, plus cropping.
Do not wait for the full moon. Full moon light hits the surface head-on, so craters cast no shadows and the face looks flat. A few days either side of first or last quarter, the sun rakes across the terminator, the line between light and dark, and craters stand out in relief.
Photographing the moon in a landscape
A lone moon in a black sky is a technical exercise. A moon rising behind a city skyline or over a pier is a photograph, and the difficulty is exposure: after dark, the moon is many stops brighter than anything on the ground.
The solution is timing. The moon rises roughly 50 minutes later each day on average. On the evening before a full moon it typically rises shortly before sunset, so it comes up into a sky that is still bright, while the landscape still has twilight on it. One exposure can hold both. Planning apps show the exact time and direction of moonrise from any spot, which lets you line it up with a lighthouse, a ridge or the CBD skyline from across the bay.
For a big moon over a small landmark, stand well back and use a long lens. The moon's size stays constant while the landmark shrinks with distance, so the further back you are, the larger the moon appears against it.
Use a tripod, a two-second timer or remote release, and turn image stabilisation off on the tripod unless your camera's manual says otherwise. Near the horizon, the air ripples with heat and haze, softening detail. For the sharpest moon, wait until it is higher in the sky.
The moon is a grey rock lit by direct sunlight. That single fact explains most failed moon photographs.
How to photograph star trails
Star trails record the Earth's rotation. Over a long exposure, each star draws an arc across the frame, and those arcs circle the celestial poles. The sky turns about 15 degrees an hour, so thirty minutes gives short strokes and an hour or two gives strong curves.
In Australia, the circles centre on the south celestial pole. It sits due south, at an elevation equal to your latitude, which is roughly 38 degrees above the horizon from Melbourne. Unlike the northern hemisphere, there is no bright pole star to mark it. The traditional method is to extend the long axis of the Southern Cross about four and a half times its length. A planning app is quicker. Point south for concentric circles; point east or west and the trails become long, gently curved diagonals.
Star trails are more forgiving of light pollution than the Milky Way, because bright stars still record strongly against a lighter sky. The outer suburbs, the Mornington Peninsula or the foothills of the Dandenong Ranges can all work. A strong foreground, a tree, a hut or a jetty, gives the circles something to turn around.
Star trail settings and stacking
You can make star trails with one very long exposure, but most photographers now stack. The camera shoots a continuous sequence of shorter exposures, and software combines them, keeping the brightest pixel from each frame. Stacking controls noise and heat, lets you drop frames spoiled by aircraft lights or passing headlights, and means a bumped tripod ruins one frame rather than the whole night.
Set the camera to manual, focus on a bright star in magnified live view, then tape the focus ring. Use a built-in interval timer or a remote intervalometer, with the interval set a second longer than the exposure so there is almost no gap between frames. Turn off long exposure noise reduction, which makes the camera pause after every frame and leaves gaps in the trails. Free stacking software is widely available, and Photoshop's Lighten blend mode does the same job.
A session of an hour or more drains batteries, particularly on a cold night, and lets dew form on the lens. A USB power bank feeding the camera, where supported, and a lens heater strap solve both.
The limits of the 500 rule
The 500 rule, 500 divided by focal length for the longest exposure before stars trail, is only for pinpoint stars. It does not apply to star trails, where trailing is the point, or to the moon, which needs fractions of a second.
Even for pinpoint stars it has blind spots. It ignores sensor resolution, so on a modern high-megapixel camera viewed at full size, its exposures often show short streaks. It ignores aperture, which the NPF rule accounts for. And it ignores direction. Stars near the south celestial pole move far more slowly across the frame than stars on the celestial equator, low in the east and west. A frame aimed south can tolerate a noticeably longer exposure than one aimed east with the same lens.
Treat any rule as a starting point. Shoot a test, zoom to 100 per cent, and let the picture tell you. The night photography workshop at Flinders Street is a good place to build that habit, and a private lesson can cover interval timers and stacking on your own camera. For dark skies further afield, our 2027–28 photography tours go to regional locations.
Practise this
- Photograph the moon at first quarter using the Looney 11 rule, then at one and two stops brighter. Compare crater detail along the terminator.
- Find the south celestial pole from your backyard using the Southern Cross, then check it against a planning app.
- Shoot a 45-minute star trail sequence facing south, stack it, then make a second one facing east and compare the shapes.
Questions, answered.
What settings should I use to photograph the moon?
Why is the moon just a white blob in my photos?
How long do you need for star trails?
Where do star trails circle in Australia?
Published by CapturEDU, Melbourne photography education. Our educators are award-winning photographers with a Master of Arts in Photography (RMIT), AIPP and RPS membership.
