The depth of field of an image is, put in a simple way, a measure of how much of it is in focus.
For example, a deep depth of field would be a landscape photo where everything appears to be shard and in focus. On the other hand, an image with a shallow depth of field has very few elements in focus. One of the best examples of a shallow depth of field corresponds to portraits, where the subject is in focus but the rest of the picture isn’t.
Technical definition of depth of field
Let’s get more into details. The true definition of the depth of field is the distance between the nearest and the farthest points of acceptable focus.
What’s acceptable focus? It’s when a point appears to be in focus to our eyes but technically isn’t.
So the depth of field measures the distance between the nearest and farthest points that appear to be in focus to our eyes.
Changing the depth of field
The easiest way to change the depth of field is by adjusting the aperture of the lens. This changes the amount of light that goes through it and into the camera sensor.
For a given apertures (f-stop) you can change the depth of field by increasing or decreasing the magnification of the image on your sensor.
However, there are other ways to do it. It’s possible to change the depth of field by moving closer -or farther- from the object, or changing the focal length.
Actually, the depth of field is determined by a very complex system of equations and rules. If you’re interested in the science of photography, you should look into it. Otherwise, you’ll be ok just trying out different apertures. Read more about about changing the depth of field on canon camera by following this link