Sunny 16 Rule.

In photography, the sunny 16 rule (or, less often, the "sunny f/16 rule") is a method to estimate correct daylight exposures without using a light meter.The basic sunny 16 rule, applicable on a sunny day, is this:

• Set aperture to f/16 and shutter speed (reciprocal seconds) to ISO film speed. For example, for ISO 100 film, choose shutter speed of 1/100 second (or 1/125 second)

The elaborated form of the sunny 16 rule for more general situations is:

1. Set the shutter speed to the setting nearest to the ISO film speed
2. Set the f-number according to the table below:

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
f/4	Sunset

For example, to shoot ISO 100 film in sunny conditions, set the shutter speed to 1/100 or 1/125 and the f-stop to f/16. With ISO 200 film, set the speed to 1/200 or 1/250. For ISO 400 film, 1/400 or 1/500. As with other light readings, the shutter speed can be changed, as long as the f-number is compensated. For example, 1/250th of a second at f/11 would be equivalent to 1/125th at f/16.