So we know that the aperture chosen will determine how much light enters the camera. 1.8 will let in a lot of light, while 22 will let in much less. This is easy to understand when considering extreme situations: interior shots with artificial lighting on one end, a sunny landscape on the other. But the other reason to choose the size of the aperture is based on desired depth of field.

Depth of field is the amount of focus an image has. A lens can only focus on one plane or distance at a time. You can increase your plane of focus to be very deep, as in a landscape where the foreground and the background, the trees and the grass are all sharp; or you can decrease your plane of focus to make it shallow, focusing on a single flower and allowing the background trees to blend into what are “circles of confusion,” or “bokeh.”

Do you ever wonder how a photographer achieves the effect of getting something in the foreground in very sharp focus while the background is blurred into a painterly effect like this:

This is called shallow depth of field, and it is achieved by opening the aperture nice and wide to let in a lot of light, anywhere from 1.2 – 5.6. The results will be reflective of a spectrum – 1.2 being the shallowest depth of field and graduating upwards.

Another method of achieving shallow depth of field is to use a longer lens, telephoto or zoom lens, like 135mm, and to focus on the subject very closely.

The opposite effect, getting the foreground and background in focus, such as the color photo here is achieved through making the aperture smaller, anything from 11 – 32 and on.

An easy way for an SLR user to test what their depth of field will look like in the final image (especially important for film users, but also important for digital cameras if you are trying to capture something quickly) is a feature that many modern SLR cameras come equipped with, a small button near the lens called the “depth of field preview.” After setting your aperture, holding down this button will allow you to preview the amount of depth of field achieved by those settings. The preview image will be darker than normal, so look carefully and learn the principles through trial and error as well. Once you have them memorized and practiced, you will be able to know just by the numbers what kind of effect your aperture will have.

Once you have the idea of aperture reasonably understood, you must also know that the key to using these principles of depth of field, and manual aperture in general, is also hinged upon the use of the shutter speed. If you are out in a bright area and want to achieve a shallow depth of field, you will need to open up your lens very wide. So to prevent overexposure, you will also need to adjust your shutter speed to be very fast. Conversely, if you want everything to be in focus, but don’t have enough light, then you will need to adjust your shutter speed to be longer (and you might need a tripod as well.)

Read more about Shutter Speed