Kirk Marshall Photography

Depth of field is defined as the distance between the nearest and farthest objects that are in acceptable focus. Depth of field depends on four factors: the focal length, the distance to your subject, the aperture and the circle of confusion. 

The circle of confusion is a characteristic of your camera/lens and can’t be altered so although we need to take it into account to determine the depth of field we can effectively ignore it for the rest of this discussion.

Mathematically, the depth of field is directly proportional to the distance to your subject; which means that as the distance to your subject increases so does the depth of field. For example: focusing on a city skyline will have a greater depth of field than will focusing on a flower (assuming other factors remain constant). 

Conversely, the depth of field is inversely proportional to both the size of the aperture and the focal length. Therefore, an increase in the size of the aperture (i.e. smaller f-number) decreases the depth of field and a decrease in the size of the aperture (i.e. larger f-number) increases the depth of field. Also, as the focal length increases, the depth of field shrinks. If you’re using a zoom lens (e.g. 24-70mm) moving from the wide end (24mm) to the telephoto end (70mm) will result in a reduced depth of field. 

While these statements will help you understand the basic fundamentals of depth of field and help you achieve basic goals when shooting, sometimes you need more precision. Rather than carry the mathematical formula and a calculator in your camera bag you can easily determine the depth of field for a particular set of settings using a smart-phone app. A quick search in the app store will list dozens. 

(Canon 5DMII, EF100 Macro, 1s, f/11, ISO 400)