The aperture is a variable opening situated at the base of the lens close to where it connects to the camera body. As light enters the camera it is the first control over how much light reaches the sensor. If your camera was your eye, then the aperture would be your pupil. In the same way a pupil dilates and contracts to let more and less light in so too does the the aperture open and shut to control how much light passes through the lens.
The amount of light that the aperture lets in is set using a scale known as f/stops. The smaller the number of the f/stop, the larger the aperture and the more light it lets in. For example f/2.8 is a larger aperture than f/4. In fact f/2.8 lets in twice as much light as f/4, and this difference is known as a stop of light. Most DSLR’s will allow you to set the aperture in 1/3 stop increments.
Size is more than a number
Something that might not be immediately obvious is that the f/stop number does not refer to the size of aperture in terms of mm’s or inches. It actually refers to the ratio between the size of the aperture and the focal length of the lens. This is why the number gets larger as the hole gets smaller and why f/2.8 on a small lens, like a camera phone, will not be the same size as f/2.8 on full size DSLR, but they will both let in the same amount of light in relation to the lens that they are part of and will result in the same exposure.
So the good news is that no matter what camera you use you will get consistent results in exposure when your f/stop, shutter speed and ISO are the same.
When you change the aperture, shutter speed or ISO as well as altering the exposure, you will also change a creative aspect of your final image.
In the case of the aperture, changing the size also changes something known as the depth of field, which basically means how much of the image will be in focus. The smaller the aperture the more of the photograph will be in focus and conversely if you have your aperture wide open, then only a very specific part of the photograph will be in focus. For example:
[Image showing comparison of different aperture settings on depth of field]
You can use the effects of depth of field for creative effect, for example if you want your subject to stand out you would use a large aperture (small f/number) to create a shallow depth of field which will blur the foreground and background and isolate your subject and naturally draw in the viewers eye.
Conversely, if you wanted to create a landscape or even an optical illusion where the foreground and background appear to be on the same focal plane, you would need to use a small aperture (large f/number) to create a deep depth of field.
There are a few other factors involved; the length of the lens, the size of the sensor and the distance to the subject, but generally speaking the rule holds true, the larger the aperture the smaller depth of field. However, if you want to get more precise about calculating the depth of field, try this great app by Dennis van den Berg, it’s really well designed, has an extensive library of cameras and lenses and is currently £1.99 from the Apple App store. If you prefer things free, then this website has a handy interactive table.
Finally, if you want to dig deeper for a really comprehensive explanation of depth of field, then this video will be right up your street!