What is aperture?  

The Basics:

An Aperture is the opening within a lens that controls how much light hits the imaging sensor. Think of it as the pupil in your eye.

The size of the aperture will dictate two things; firstly, your depth of field and secondly, the shutter speed required to expose the image correctly. Adjusting your Aperture settings will therefore play a huge part in the look and feel of the image.

A Little More Detail:

In photography, the size of the Aperture is expressed in ‘F numbers’, for example:

F/1.4, F/2.0, F/2.8, F/4.0, F/5.6, F/8.0, F/11.0, F/16

On the face of it, F numbers seem like quite a random sequence of numbers, but actually represent the diameter of the Aperture as a ratio to the focal length of the lens. Therefore, the smaller the number, the larger the Aperture, and of course the larger the Aperture, the greater the amount of light which is able to pass through.

As with shutter speeds, F numbers are listed as ‘stops’ which have a halving or doubling relationship to each adjacent number. For example, F/2.8 lets in twice the amount of light as F/4.0, but only half as much as F/2.0. This of course has a critical effect on exposure; if you ‘stop down’ from F/2.8 to F/4.0, you will need to keep your shutter open for twice as long, in order to let in the right amount of light.

So why would you adjust your Aperture speed? Broadly, there are two reasons why you might adjust your Aperture. 

Firstly, you might change your Aperture to adjust your depth of field, ie the amount of the image which appears in focus. This of course will play a huge part in how your image turns out. For example, portrait shots tend to be captured with a large Aperture, which will blur the background, making the face stand out more. Likewise, a landscape shot might be captured with a small Aperture, so that everything is in focus. To help you remember the relationship between F number and depth of field, think of a small F number (ie, a larger Aperture) as giving you a small or shallow depth of field. Likewise, think of a large F number (ie, a smaller aperture) as giving you a larger depth of field.

Secondly, you might be shooting in low light, and want to use as large an Aperture as possible, so that you can use a fast shutter speed, thus avoiding camera shake blur. The larger the aperture a lens is capable of being set to, the faster the shutter speed you can use – this is why you’ll often hear a lens referred to as being ‘fast’. That is to say, typically, large aperture lenses are more expensive than smaller aperture lenses, as it is technically more complex to create a large aperture lens.

So how do you know what Aperture a lens is capable of being set to? This information is always found in the naming convention of a lens, as well as being printed on the lens itself. For example, the EF 85mm F1.2L II USM can be set to a maximum of F/1.2. A zoom lens will often support a different maximum Aperture, depending on the focal length. Therefore, the lens will quote the maximum aperture size at its widest angle, to the maximum aperture size at telephoto. For example, the EF-S 18-135mm f/3.5-5.6 IS STM can be set to F/3.5 at its widest angle, or F/5.6 at 135mm.