There are many ways to effect the depth of field. Depth of field is the distance that is in focus. This post is going to demonstrate how aperture changes the depth of field.
Aperture is one of the easiest ways to change the depth of field and that is why I am going to talk about it first. The aperture is the size of the opening in the lens that lets how much light goes into the camera. Aperture is measured as a ratio and common ratios include f2.8, f4, f5.6, f11 and f16. For one reason or another newcomers to photography can find this difficult to remember so I recommend thinking of it like this. A small aperture number means a small depth of field, a large aperture number means a large depth of field.
As usual, you can find the gory technical details on aperture on widipedia. We are going to focus on how it affects your composition, and in this regard it is quite easy.
Lets take a look at this photo taken with an aperture of f4.5
If we take a look at the plant closest to the camera we can see that only the leaf in the middle of the plant is completely in focus. The leaf closest to the camera is a little blurry and some of the leaves that are farther away also start to get blury. The plants in the background also have a blurry look to them.
Now lets look at a photo of the same subject, this time with the aperture set to f16.
Now the plant closes to the camera is completely in focus with none of it being blurry. Furthermore the other plants in the background, while not in focus, are more more defined and we can see some of their detail.
So how do we use this in the composition of our photos? If we want the subject of our photos to be separated from what is going on around them, like a person on a busy street, then we can choose an aperture with a small number like f5.6, f4, f2.8 etc to separate the subject from the background. If the subject of our photo is a landscape we may want everything in the photo to be in focus all the way to the horizon. In this case we take an aperture with a large number like f16 or f22.
And that is all there is too it, the smaller the number the less of the photo is in focus, the larger the aperture number, more of the photo will be in focus.
Remember though, that aperture is just one way to manipulate depth of field. Check back later to see other ways to change the depth of field in a photo.