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.

The basic rule is, '"For astronomical photos of the Moon's surface, set aperture to f/11 and shutter speed to the [reciprocal of the] ISO film speed [or ISO setting]."

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.[1][2][3]

Moonlight photography

Moonlight photography (taking pictures of the surface of the Earth as illuminated by moonlight) is very different from lunar photography (taking pictures of the surface of the Moon as illuminated by sunlight). The Moon has an effective albedo of approximately 0.12, comparable to fresh asphalt. Since it is essentially a dark rock in full sunlight, photographing its surface needs an exposure comparable to what a photographer would use for ordinary mid-brightness surfaces (buildings, trees, faces, etc.) with an overcast sky.

The sunlight reflected by the 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..., full-moon photography requires 18 stops more exposure than sunlight photography, for which the sunny 16 rule is the guideline.[4]

Reciprocity failure

Imagine a full daylight exposure of 1/100 second at ISO 100 and f/16 (the baseline of sunny 16). Adding 18 stops to convert from the Sun to the Moon could result in a shutter speed of 8 seconds at ISO 400 and f/2 (+10 stops of time, +2 stops of ISO, +6 stops of aperture). However, on most chemical film, such an exposure would turn out too dark. This is because film does not expose in linear proportion to the light it absorbs, an effect called reciprocity failure. At light levels as dim as moonlight, it needs more light than a linear extrapolation of daylight values would suggest.

For example, testing shows[5] that Kodak Portra needs 1 extra stop for a nominal 8 second exposure, so in this case it would need 16 seconds.

In practice, moonlight photography often uses exposures of several minutes. Digital cameras generally have less reciprocity failure, but do show image noise in low light.

See also

References

  1. ↑ Jerry Lodriguss. A Beginner's Guide to DSLR Astrophotography.
  2. ↑ Jerry Lodriguss. A Guide to Astrophotography with Digital SLR Cameras.
  3. ↑ Jerry Lodriguss. A Guide to DSLR Planetary Imaging.
  4. ↑ Lance Keimig. Night Photography: Finding your way in the dark.
  5. ↑ http://www.largeformatphotography.info/forum/showthread.php?106441-Portra-160-and-400-Reciprocity-Failure

External links

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