F16 Rule

The F16 rule is not a regulation for the Lochhead Fighting Falcon but a basic light estimation method.
Commonly known as the Sunny f16 rule (also known as the Sunny f/16 rule) it can achieve correct exposure of difficult subjects. As the rule is based on incident light, rather than reflected light as with most camera light meters, very bright or very dark subjects are compensated for. The rule serves as a mnemonic for the camera settings obtained on a sunny day using the exposure value (EV) system.

The basic rule is, “On a sunny day set aperture to f/16 and shutter speed to the ISO film speed.” For example:

* On a sunny day and with ISO 100 film in the camera, one sets the aperture to f/16 and the shutter speed to 1/100 or 1/125 second (on most cameras 1/125 second is the available setting nearest to 1/100 second).

* On a sunny day with ISO 200 film and aperture at f/16, set shutter speed to 1/200 or 1/250.

* On a sunny day with ISO 400 film and aperture at f/16, 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/11 gives equivalent exposure to 1/125 second at f/16.

An elaborated form of the Sunny 16 rule is to set shutter speed nearest to the reciprocal of the ISO film speed and f-number according to this table:[2][3]

Aperture Lighting Conditions Shadow Detail
f/16 Sunny Distinct
f/11 Slight Overcast Soft around edges
f/8 Overcast Barely visible
f/5.6 Heavy Overcast No shadows

taken from http://en.wikipedia.org/wiki/Sunny_16_rule