Looney 11 rule
In lunar photography, the Looney 11 rule (also known as the Looney f/11 rule) is a method of estimating correct exposures without a light meter. For daylight photography, there is a similar rule called the Sunny 16 rule.
- With ISO 100 film / setting in the camera, one sets the aperture to f/11 and the shutter speed to 1/100 or 1/125 second (on some cameras 1/125 second is the available setting nearest to 1/100 second).
- With ISO 200 film / setting and aperture at f/11, set shutter speed to 1/200 or 1/250.
- With ISO 400 film / setting and aperture at f/11, set shutter speed to 1/400 or 1/500.
As with other light readings, shutter speed can be changed as long as the f-number is altered to compensate, e.g. 1/250 second at f/8 gives equivalent exposure to 1/125 second at f/11. Generally, the adjustment is done such that for each step in aperture increase (i.e., decreasing the f-number), the exposure time has to be halved (or equivalently, the shutter speed doubled), and vice versa. This follows the more general rule derived from the mathematical relationship between aperture and exposure time — within reasonable ranges, exposure is proportional to the square of the aperture ratio and proportional to exposure time; thus, to maintain a constant level of exposure, a change in aperture by a factor c requires a change in exposure time by a factor 1/c² and vice-versa. Steps in aperture correspond to a factor close to the square root of two, thus the above rule.
The intensity of visible sunlight striking the surface of the moon is essentially the same as at the surface of the earth. The albedo of the moon's surface material is lower (darker) than that of the earth's surface, and the Looney 11 rule increases exposure by one stop versus the Sunny 16 rule. Many photographers simply use the f/16-based Sunny 16 rule, unmodified, for lunar photographs.
Moonlight photography (taking pictures of the surface of the earth as illuminated by moonlight) differs considerably from lunar photography (taking pictures of the surface of the moon as illuminated by sunlight).
The sunlight reflected by the full or nearly full Moon onto the Earth is about 250,000 times dimmer than the light we get directly from the Sun in the daytime. Since log2(250,000)=17.93, moonlight photography in such conditions requires 18 stops more exposure than the Sunny 16 rule. Moonlight photography often uses exposures on the order of minutes. With chemical films, reciprocity between aperture and exposure settings starts to break down at low light levels. In these cases, reducing the aperture by one stop requires the exposure time to increase by more than one stop. The external links cited provide guidance on how to compensate for reciprocity failure. Note that compensation will differ for film versus digital cameras.
- Jerry Lodriguss. A Beginner's Guide to DSLR Astrophotography.
- Jerry Lodriguss. A Guide to Astrophotography with Digital SLR Cameras.
- Jerry Lodriguss. A Guide to DSLR Planetary Imaging.
- Lance Keimig. Night Photography: Finding your way in the dark.