Star Trails and Moonlight Photography

The most natural existing light at night comes from the moon and stars, and from the natural luminosity of the atmosphere itself. This latter is only noticeable in places that are completely free from artificial light at night. Even 300km (200 miles) from a large city or 50 km from a small town, the reflective glow of its street lights can be seen close to the horizon, masking the air-glow in that part of the sky. However, if truly dark places can be found beautiful then intriguing images are possible.

Among the easiest night-time photographs are star trails, which reveal the motion of the earth spinning in space by the tracks of stars across the sky. The idea is simple in principle but there are many subtleties. The camera is focused on infinity, fixed to a tripod (or laid on its back), and the shutter left open for as long as required. There is no “correct” exposure in these circumstances; in general the longer the exposure time, the longer and more numerous are the star trails. Pointing the camera east or west produces more or less straight trails at an angle to the horizon that reflects the local geographic latitude. Pointing the camera north (in the northern hemisphere) or south in the southern hemisphere produces circles around the north or south celestial poles. The camera records more star trails in this direction, because toward the celestial pole the star images are tracking ever more slowly across the film or sensor.

If the sky is bright because of light pollution or the moon, stopping down the lens affects the sky more than the star trails, all but the faintest of which are overexposed. De-focusing the lens in a series of steps during the exposure spreads the star images on the film or detector, reducing the effective exposure and revealing their colors.

The simplest equipment for star trail photography is a mechanical film camera that is not battery operated. It should be loaded with 200-400 ISO reversal (slide) film, or 400-800 ISO film with larger formats. The camera should be fitted with its standard focal length lens and operated at approximately f/4. It is possible to make star trail pictures with digital cameras, but creating a single and very long exposure is still not easily achieved, so a series of shorter exposures must be combined using image processing software to achieve a seamless result.

For photography by moonlight with the full moon overhead, illuminance is merely 450,000 times (about 18.6 stops) fainter than the sun’s illuminance. For a variety of reasons to do with the nature of moon’s surface, its illuminance dims more rapidly and in a non-linear fashion from the time of full moon. The normally “unnoticed” blue sky produced by the full moon is just as effective at filling the shadows as is the daytime blue sky. However, if moonlit exposures are longer than a few minutes, the moon’s movement across the sky will increase its effective angular diameter so that shadows running roughly north to south will appear softer. Moonlight has a slightly lower effective color temperature than sunlight (4100K vs. 5500K) so its light is slightly yellow compared to sunlight.

With an ISO 200 film and brilliant sunlight overhead an exposure time of 1/1000 seconds at f/5.6 might be indicated for an average daytime landscape. With the full moon overhead an exposure time of about 450 seconds (about 7.5 minutes) would be calculated from the above information. However, film suffers from low intensity reciprocity failure (LIRF), so is less efficient at exposures longer than a few seconds. LIRF varies from film to film, and with exposure time, but manufacturer’s data sheets often give exposure factors to compensate for LIRF. However, LIRF also varies with exposure temperature, so exposures will be shorter on winter nights. LIRF may affect the separate layers of color film differently, giving rise to difficult-to-correct color balance shifts.

Many of these problems vanish with digital cameras; however, not all such cameras are able to take adequately long exposures and when they do the images are often “noisy.” The long-exposure detector noise in digital cameras will usually be lower under very cold conditions (but so will battery efficiency).