Photography 101: What is an "aperture"?

Aperture is one of the three main controls you can use when you are taking a picture. Along with shutter speed and ISO, aperture controls how light will hit the sensor or film.

Aperture is referred to the lens diaphragm opening inside a photographic lens. The size of the diaphragm opening in a camera lens controls the amount of light that passes through onto the film inside the camera the moment when the shutter curtain in camera opens during an exposure process. The size of an aperture in a lens can either be a fixed or the most popular form in an adjustable type (like an SLR camera). Aperture size is usually calibrated in f-numbers or f-stops. i.e. those little numbers engraved on the lens barrel like f/22, f/16, f/11, f/8.0, f/5.6, f/4.0, f/2.8, f/2.0, f/1.8.

The larger the hole the more light that gets in - the smaller the hole the less light. "Stopping down" the lens from f/4 to f/5.6 will halve the amount of incoming light. "Opening up" the lens from f/5.5 to f/4 will double the amount of light.
Stopping down means to reduce the size of the iris of a lens. This increases the depth of field of the image, but results in dimmer images at the film plane. You can can compensate for this by increasing the exposure time (shutter speed),or increasing the ISO. Keep in mind that a change in shutter speed from one stop to the next doubles or halves the amount of light that gets in also - this means if you increase one and decrease the other you let the same amount of light in - very handy to keep in mind).



The best way to get your head around aperture is to get your camera out and do some experimenting. Go outside and find a spot where you’ve got items close to you as well as far away and take a series of shots with different aperture settings from the smallest setting to the largest. You’ll quickly see the impact that it can have and the usefulness of being able to control aperture.