What are apertures and why are they important?

To a photographer, the aperture is the opening in the lens through which light passes into a camera. It's most important physical property (okay, it's sole physical property of any interest!) is it's size, because that has a number of rather important effects on the pictures you make.

First of all, I'll be specific about what I mean and don't mean by the 'size' of an aperture. I don't mean a simple measurement across the opening. I do mean a measurement of that opening as a proportion of another physical property of the lens, it's focal length.

On most lenses, the maker will mark the focal length on the front. A typical standard lens on a traditional 35mm SLR might have '50mm f/2.8' engraved around the front, for example.

So, what does that all mean? It means the lens has a focal length of 50mm and a maximum aperture of f/2.8, which is the same as saying 50mm/2.8, which is roughly 18mm.

Fascinating, eh? No, not really. Unless you're actually fascinated by numbers.
The point is not the number itself, but what that effect that has on your pictures.

The aperture affects how fast light comes through the hole - a bit like turning on a tap. Open it wide and water (or light) comes streaming through. Keep it small and water is reduced to a trickle. This helps control the amount of exposure on your film or digital sensor. Using the water analogy again, getting the exposure right is like filling a bucket from a tap. You need to fill it right to the brim, not half full, but you also don't want it to overflow. You can do that by opening the tap wide for a short time, or leaving it dripping for a much longer time. To get a full bucket, it doesn't really matter which way you do it. Exposing your film or sensor is the same. You can let in light very fast for a short time, or slower for a longer time. So, a big aperture means we can have fast exposures. A small aperture means we need slower exposures.

The opening also affects 'depth of field'. Big depth of field means you can keep lots of things 'in focus' at different distances at the same time. Small depth of field means you can keep just one thing in focus at one distance and let things at other distances blur. More about that another time...

So, use the aperture to control exposure. That's a technical use. Use the aperture to control depth of field. That's potentially a creative use as it lets you control how people see the things in your pictures.

Last thing for today - why not just give the aperture as a straightforward measurement? Why give it as a proportion of the focal length - what's often referred to as an 'f-number'? The answer is to do with an interesting and very useful property of lenses. Any lens at the same f-number lets light pass at exactly the same rate. That means that it's very easy to measure the exposure as a combination of an 'f-number' and a time, because it doesn't matter which lens is used.