# Sunny 16 rule

In photography, the sunny 16 rule (also known as the sunny f/16 rule) is a method of estimating correct daylight exposures without a light meter. (For lunar photography there is a similar rule known as the looney 11 rule.) Apart from the obvious advantage of independence from a light meter, the sunny 16 rule can also aid in achieving 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 [reciprocal of the] ISO film speed [or ISO setting] for a subject in direct sunlight."[1] For example:

• On a sunny day and with ISO 100 film / setting in the camera, one sets the aperture to f/16 and the shutter speed (i.e. exposure time) to 1/100 or 1/125[2] seconds (on some cameras 1/125 second is the available setting nearest to 1/100 second).
• On a sunny day with ISO 200 film / setting and aperture at f/16, set shutter speed to 1/200 or 1/250.
• On a sunny day with ISO 400 film / setting 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. More in general, the adjustment is done such that for each stop in aperture increase (i.e., decreasing the f-number), the exposure time has to be halved, and vice versa. This follows the more general rule derived from the mathematical relationship between aperture and exposure time—within reasonable ranges, exposure is inversely 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. I change in the aperture of 1 stop always corresponds to a factor close to the square root of 2, thus the above rule.

An elaborated form of the sunny 16 rule is to set shutter speed nearest to the reciprocal of the ISO film speed / setting and f-number according to this table:[3][4]

Tessina with exposure guide plate from the 1960s. At that time, DIN 21 was equivalent to ASA 80. After 1983, DIN 21 was ASA 100.[5] On this guide plate, DIN 21 uses f/16 and 1/125, consistent with Sunny 16.
f/22 Snow/sand Dark with sharp edges
f/16 Sunny Distinct
f/11 Slight overcast Soft around edges
f/8 Overcast Barely visible

## Example

Sunlit subject shot on a digital camera set to ISO 100, exposed at f/8 at 1/400 second, which is the same exposure value as f/16 for 1/100 second

Rolleiflex TLR exposure guide

This is a scene of average brightness, in direct sunlight. It was shot at ISO 100, f/8 at 1/400 second – the recommended "sunny 16" exposure – which is what autoexposure gave.