The aperture is the hole in the lens – it is a variable diaphragm that can be made larger or smaller, controlling how much light reaches the sensor in your camera.
You can control the aperture size using the dial on your SLR (or it can be set for you by the camera). The aperture size is measured on the f-stop scale. The relationship between the numbers on the scale can be hard to grasp. If you think of the f/stops as fractions, with f/4 being twice as large as f/8, f/8 twice as large as f/16, and so on.
Your choice of aperture will vary depending on the lens you use, but it will most lenses range from a widest setting of around f/4 to a narrowest setting of around f/22.
Aperture size is also divided into stops – f/4, f/5.6, f/8, f/11, f/16, f/22, and so on – with each stop effectively halving the amount of light reaching the sensor. You can compensate for the amount of light reaching the sensor by doubling the exposure time if needed.
Depth Of Field
As well as helping to control the exposure, your choice of aperture also affects the ‘depth of field’ (or DoF).
The depth of field is a measure of how much of your photo is in focus, both in front of, and behind, the point you’ve actually focused on. Depth of field is more apparent when your image contains elements at varying distances from the camera, and is particularly noticeable in the background. Whether your background is sharp or out of focus depends on your aperture choice.
This image of the poppies - if you look at the image, just the poppy standing out from the crowd and the poppies in line with it are in focus - this image has a small depth of field - it has an f/stop of F4. If I had used an f/stop of say f22 the poppies going off into the distance would also be in focus. To give you an idea of how you can use depth of field/ your f stops for creative effect here is another image.
Once again here I have used a small depth of field - meaning that I have a small area of the photograph in focus this was shot at f4 again, which means that just the Sally lightfoot crab is in focus and the volcanic rock behind and in front goes into soft focus so that we are not distracted by it, but our attention is kept on the crab and it's amazing colours.
Tomorrow we will cover some more ideas about composition and the rule of thirds.