By Mark Blezard
Your aperture settings serve two purposes. The first is exposure, the bigger the opening (f/2.8 for example) the more light is available. The second function is focus. The smaller the aperture (f/16 for example) the greater the range of focus you have. This is known as 'depth of field.'
Modern cameras will allow you to compose your image whilst the lens is fully open, to make it easier to see through the lens. However, if your camera is telling you that it will select f/16 when you make the shot, the object that you've fixed on will not be the only object in focus. More of the foreground and background will also be sharp because you have a greater depth of field.
Take a look at this image. I wanted to draw attention to one single branch of the tree, not all of it. So, I used a large (f/2.8) aperture in order to reduce the depth of field. Had I used an f/16 or smaller, the branches behind would have come into focus and therefore making it difficult to distinguish one branch from another.
Remember, the best way to manage your depth of field is by taking your camera out of automatic exposure mode and selecting aperture priority setting. Let your camera select the shutter speed according to your preferred aperture.
