F stop

The lens iris is used to control the amount of light transmitted and is calibrated in f stops. With a high number f stop such as f16, a small amount of light is allowed to pass through the iris to the imaging device. A low number f stop such as f 2.8 allows a larger amount of light to pass. A lens iris is typically calibrated in the following f stops: f1.4, 2, 2.8, 4, 5.6, 8, 11, 16, and 22. Each higher f stop cuts the amount of light by exactly half.

Lenses can also be calibrated in T stops.