The light of the moon is made up of the same colors as daylight, but it is very much weaker (the sun is about 600,000 times more brilliant than the full moon). So the only real difficulty about taking photographs by moonlight is the length of the exposure. Even with a fast panchromatic emulsion and a wide aperture lens, pictures of open landscapes call for exposures of fifteen minutes or more. The apparent bluish color of moonlight is due to the fact that in dim light the peak sensitivity of the eye is at a slightly different point of the spectrum.
During an exposure as long as fifteen minutes, the image of the moon moves an appreciable distance, and if it is included in the picture area, it comes out as a long white sausage-shaped blur on the print. (Any stars in the picture area will appear as lines of light and not as points.) This is no drawback if the moon itself is kept out of the picture, but in that case there is a danger that the resulting photograph will be merely a flat and toneless view without anything to indicate that it was taken by moonlight.
There are several photographic methods of including an image of the moon in a moonlight picture. The simplest method is to photograph the scene just after sunset a few days before full moon when the moon rises early in the evening. The exposure for the twilit landscape is about right for the moon itself so the moon may be included in the picture, and photographed at the same time as the scene.
Other methods rely on making two separate exposures; one for the landscape from which the image of the moon is excluded, and the other for the moon in which the exposure is so short that even if a part of the landscape is included, it does not register. The negative of the moon may be made by a second exposure on the same film or plate as the landscape, or it may be made separately and printed in afterwards like a cloud negative. The advantage with a combination print is that the same negative of the moon can be used for a number of different landscapes; the position of the shadows in the scene should, however, match the position in which the moon is printed.
If the moon is photographed with a normal angle lens, the resulting image is apt to look disappointingly small. For the picture to look natural, the image of the moon must be enlarged up to three times the size that it would normally appear in the print. This can be done by making the exposure for the moon through a long focus or telephoto lens with a power of 2x or 3x , while the exposure for the landscape is made through a normal angle lens. When the two negatives are combined and printed, the eye accepts the result as natural.
Alternatively, the moon may be taken through the normal lens, an enlarged duplicate negative made, and the latter combined with the landscape negative.
Open landscapes under a full moon need an exposure of about 2-5 minutes at f3.5 on the fastest panchromatic emulsion. The light given by the half-moon is less than half of the light of the full moon; exposures under the half-moon should be at least three times the full moon figure-i.e., 6-15 minutes at f3.5 on fast panchromatic film.
The above exposures will generally produce very dark shadows which help to convey the atmosphere of a moonlit scene. If the picture is to show the same amount of shadow detail as a daylight photograph, the exposures must be multiplied by four.
Any camera that will take a time exposure is suitable for making pictures by moonlight. It should be mounted on a very steady tripod because of the long exposure times that have to be given. Preferably the camera should have both normal and long focus lenses so that the moon can be photographed separately on an enlarged scale for combination printing.
The lens aperture should be at least f3 or the exposure times will be inconveniently long.
The effect of stray lights and reflections is greatly increased by the darkness and the very long exposure, so the lens should be equipped with a really efficient lens hood. A piece of black card is useful for holding in front of the lens while lights of road vehicles are crossing the picture area.