Understanding Depth-Of-Field

by William Lulow

We have all heard of the term “depth-of-field,” but what does it actually mean? Well, it is a term that refers to the ability of a lens (or more accurately a series of glass elements within a lens barrel) to carry focus of a subject from point A to point B within the field of view. Lenses have many characteristics. The quality of a given lens depends on the original size and quality of the glass used and the precision with which the elements are assembled in order to focus light rays on the focal plane of the camera. Telephoto lenses have elements that are fairly far apart within the structure of the lens itself. Because of the distance between these elements, these lenses have a difficult time carrying focus between areas in the subject that are also far apart. Wide angle lenses have elements that are very close together. This enables them to carry focus on the subject from very close to almost infinity. So, if you’re looking for great depth-of-field, you will need to use lenses of the wide angle variety. If you want very shallow depth-of-field, you need to use lenses of the medium telephoto to telephoto variety.

With this said, depth-of-field sometimes refers to the sharpness of the image throughout it’s focusing range. Extremely good depth-of-field usually means that the subject is in sharp focus from the front of the scene all the way to the back of the scene. With a view camera (a camera that has independent film and lens planes), infinite focus can be obtained by using the camera’s tilting front and back together. If the film plane and the lens plane’s lines can be arranged to meet at some point in a virtual distance, the image on the film plane will be in focus. The depth-of-field will be infinite. Modern digital cameras do not have this ability. The film plane line and the lens plane line are always parallel. Therefore, infinite focus is only possible with a lens of the wide angle variety.

300px-Scheimpflug

This is an illustration of what is called the “Scheimpflug Principle.” Basically, it says that when the image plane and the subject plane can be focused by the lens so that their lines intersect, “infinite focus” of the image will be achieved.

If you are shooting with a digital camera, a good wide angle lens will be able to carry sufficient depth-of-field for most photographic purposes, but with digital lenses, optimal sharpness of focus is achieved when the lens’ aperture is stopped down a bit from maximum. Therefore, the faster the lens, the more it will be able to be stopped down to yield infinite focus.

Canon 24mm f_1.4

It is important to realize that the larger the piece of glass used in making the wide angle lens (or any lens, for that matter), and, of course, the quality of the glass, determines how sharp and how much depth-of-field the lens will produce. A lens that has a maximum aperture of f/1.4 say, will produce almost infinite depth-of-field when stopped down to f/11 or f/16. A lens that only has a maximum aperture of f/3.5 probably will not produce infinite depth-of-field no matter what the aperture. The glass just won’t be able to resolve to maximum sharpness. So, if great depth-of-field is what you’re looking for, you’ll need to buy an expensive, wide angle lens. The cheaper (“kit” lenses) just don’t have the resolving power if you are looking for great depth-of-field.

So, the rule I’ve said before is to buy the fastest (f/1.4, f/1.8, etc) lens you can afford. They will typically be the best.