On all DSLRs, there is a mode dial that has letters written on it, A, M, S P, Av, TvAuto etc. In my previous post I discussed the ISO button and stated that there are three main factors, which affect the exposure of an image; ISO, aperture and shutter speed. This post is about Aperture.

Aperture is simply the size of the hole through which light enters the camera. Imagine you have two hosepipes. One fat hosepipe and one skinny one. The fatter pipe, with the big hole, will obviously let more water through. Likewise, the wider the aperture setting on your camera, the more light can enter the camera and hit the sensor.

So, if you are in a low light environment and your pictures are dark (underexposed), then opening up the aperture will allow more light to enter the camera and make the image brighter. Conversely, if you are in a really bright environment, for example, outside on a sunny day, then your image may come out very bright (overexposed). In this case, you may need to decrease the aperture and make the hole smaller. This will mean that less light can enter the camera for any given time.

When you change the aperture on your camera, you will see numbers like 1.2, 1.4, 1.8 2.8, 5.6, 8, 11, 22 etc. These are indications of the aperture size. Now, you would think that the higher the number, the larger the aperture hole but the number is a fraction, so 2.8 is actually 1/2.8 and 5.6 is actually 1/5.6. Thus, 2.8 is a larger aperture than 5.6. Let’s talk examples as I find that they always help me understand things better.

Assume that the ISO is fixed at 200 and the shutter speed is fixed at 1/100s. The only variable therefore is the aperture. You take a shot with the aperture at 1.4 If you then take a picture at 1.8, 2.8, 4.0, 5.6, 8, 11, 16 etc, you will see that these pictures get darker and darker. So, if your images come out dark, then go for the lower aperture numbers.

However as in life, it’s not quite that simple. Aperture also has another impact on an image. The larger the aperture (i.e. the lower the number) the more blurry (bokeh) the background becomes! If you look at the first three images, you can see the background is slightly sharper at 2.8 than at 1.4.

Often, this bokeh is a good thing and brings your subject away from the background and emphasises them. However, if you need more of the scene in focus, for example you are shooting a group of people, then you will need to use a higher aperture. This may of course mean that your image becomes dark. If this is the problem, then you have a few options, including: 1) Finding a brighter, more well lit environment; 2) Using flash; 3) Increasing your ISO, see the previous blog post about ISO; and 4) Changing your shutter speed, which we will look at in an upcoming post.

So, if you need more light or want to blur the background, then go for a lower aperture. Conversely, if you have too much light or want more of the scene in focus, then try a higher aperture.