Aperture is the term used to describe what is simply the hole in the lens that light travels through to reach the camera’s sensor or film. This hole can be set at different sizes, and combined with shutter speed, you get the two main settings which control exposure. There is some maths involved to get the actual values, but to be honest, that doesn’t really matter. What’s more important is to know what aperture is, and what happens when you change it. Getting your head round aperture can be a bit of a hurdle, so hopefully this will help a little bit.
The first thing to get your head round is that the smaller the number the larger the hole. OK, that may sound a little odd but in this example f2.8 is the largest hole, and f22 is the smallest hole for the lens shown in the next photo.
Don’t worry about the ‘f’ either, there aren’t ‘g’ settings or ‘z’ settings, but it’s handy to know that when someone says “I used f8″, you know what they are referring to.
There are of course a range of settings in between the ones shown, and depending on what lens your camera has may effect what settings are available to use, but this should give you an idea of what is actually happening when you change the aperture setting on your camera. By making the hole smaller, you are reducing the amount of light that reaches the sensor or film in the time that the shutter is open. So from this you can start to see how shutter speed and aperture work together.
For example, if you are getting the perfect exposure with a shutter speed of 1 second and an aperture of f8, and then it gets a bit darker. You want to let more light in, so you can either keep the shutter open for longer, or make the hole bigger. Both will let more light in, but both have their own effects on the image, and it is these effects you should learn to understand.