In photography, the aperture of a lens is the circular opening in the lens that allows light to pass through it (in physics, an aperture is any hole through which light travels, irrespective if there is an actual glass lens or not). The size of a lens aperture, usually measured in f-stops, has big implications on the resulting image - most obviously that a larger aperture diameter lets more light through, and therefore allows your camera sensor to capture more light. Just as importantly however, the size of the aperture also affects the depth of field - i.e. the range, measured in meters or milimeters, within which things appear acceptably “in focus”.

Lenses used in photography are typically described by both their focal length and their f-number (or focal ratio or f-ratio or f-stop) to indicate how “zoomed in” the resulting image is, and how large the maximum aperture can be, respectively, for a given lens. In plain english, the f-number tells you how large your aperture can get. f-numbers look like “2.8” or “5.6”. The smaller the f-number, the larger the aperture, and conversely the larger the f-number, the smaller the aperture.

For example, a lens that is marked with f/1.4 has a f-number of 1.4, and will generally allow more light to hit the sensor than a lens marked with f/5.6.

The f-number itself is a ratio of the lens focal length to the physical aperture diameter, according to the following simple equation:

f-number = focal length / aperture diameter

Example: a 50mm f/1.4 lens, according to the above formula, would have an aperture diameter of about 36mm.

The fact that the f-number is a ratio between the focal length and aperture diameter has several implications, and allows photographers to make a number of mental shortcuts.

• High f-numbers represent small physical aperture diameters, vice versa for low f-numbers.
• The conventional progression of “full stop” f-numbers increases by the square root of 2. Example: f/1, f/1.4, f/2.0, f/2.8, f/4.0….
• Each full stop represents a doubling or halving of resulting image illuminance. Example: f/1.4 is half the illuminance of f/1.
• Lenses with equal f-numbers but differing focal lengths will result in the same exposure, provided the luminance of their scenes is the same and ignoring light transmission inefficiencies. Example, a 50mm f/1.4 and 100mm f/1.4 will be equally bright.

These shortcuts (among many) allow photographers to make quick mental decisions when taking photographs, such as how to adjust their shutter speed when changing the aperture.