Aperture made easy

Aperture is the opening in your camera lens that lets in light. Together with the shutter speed, it allows you to control how much light you let into your camera. The bigger the aperture / opening in the lens the more light is let in; the smaller the opening, the less light is let in.

One of the main reasons that people struggle with aperture is the fact that the numbering system is backwards! So an f22, for example, is a hell of a lot smaller than an f5.6 or an f1.2.

It took me ages to learn why this numbering system was backwards – initially I just thought it was some strange quirk of history that had been handed down to us to make our lives difficult. This isn’t quite the case though. And while it isn’t even vaguely crucial that you remember why the numbering system is the way it is, it is quite interesting (yep, I’m a fan of QI):

The aperture value that is given on your camera expresses the ratio of the focal length to the diameter of the aperture. I can hear some of you saying “Huh?” Simply put: if you have a 100mm lens with an opening that is 50mm in diameter then your aperture will be an f2 (100mm ÷ 50mm = 2). Similarly, if you have a 24mm lens with an aperture of f16 then the actual size of your aperture would be 1.5mm in diameter (24mm ÷ f16 = 1.5mm).
Understanding why it’s backwards though isn’t really going to help you remember that it is backwards – after all, we’ve been trained since we were knee high to know that a high number indicates a larger amount: four chocolate cakes is more than one chocolate cake.

With aperture of course, the opposite is true. An aperture of f11 is a lot smaller than an aperture of f1.8. An f1.8 aperture is going to let in a hell of a lot more light than an f11.

It goes without saying that the more you practice working in AV or M mode, the more you’ll find yourself just naturally remembering how the numbering system in aperture works.

Once you get the hang of this, then it’s time to start thinking about what it is that aperture does for your images – apart from just letting in extra light. Aperture controls your depth of field. Depth of field refers to the area in your image that is acceptably sharp or in focus. You get a narrow depth of field or an extensive depth of field. The easiest way to explain this is to show it.

In the top image below there is an extensive depth of field. This is caused by having a small aperture (in this instance an f14). In the bottom image there is a limited depth of field – notice how just the woman’s face is sharp? This is caused by having a large aperture (in this instance an f2.8).
So that’s aperture in a nut shell. Obviously it still interacts with shutter speed and ISO, but if you’re shooting on aperture priority mode (AV or A) now you know what your aperture is doing – it’s creating either an extensive dof (depth of field) or a limited dof, depending on whether you use a small aperture or a large aperture.
There are other things that affect depth of field – and a while back I wrote an article on that which you can read here.

Tags: Shot of the Day