Aperture is defined as an opening in a lens’s diaphragm that allows light to pass through it. The light travels down the lens through its optics and hits a light sensitive surface (film or sensor). Apertures appear as f/ stops and are written as numbers, often with decimal points. Lower f/stop numbers (eg. f/1.4) give more exposure as they represent a larger opening in the lens’s diaphragm, while higher f/stop numbers (eg. f/16) give less exposure because the opening in the lens is smaller. This will seem confusing and contradictory at first, but think if someone says “larger aperture” they’re referring to how large the opening in the lens is, not how large the number is.

Whilst aperture controls the amount of light passing through a lens, it can also be used to control the amount of depth of field present in your images. Depth of field (DoF) is the area of “acceptable sharpness” in an image. Simply put: how blurry the area behind or in-front of the subject is.

Lower f/stop (f/1.4) - larger aperture - more light - less DoF - blurrier the background.

Higher f/stop (f/16) - smaller aperture - less light - more DoF - sharper the background.

Aperture is one of the three elements in the Exposure Triangle, along with Shutter Speed and ISO.

How to choose your aperture? Now that we know how to control depth of field and what aperture is, how do we know what aperture to use in different scenarios. For classic portraiture photography the subject is separated from the background by using “selective focus”, this is achieved by choosing a larger aperture which will give a shallower depth of field. When choosing an aperture for landscape photography we want as much information in the image as possible, to do this we want maximum depth of field which is achieved by selecting a smaller aperture.