Depth of field is one of the most important distinguishers between professional and amateur photography. Composition and exposure might be easy to grasp, but controlling your focus is harder, mainly because there are so many ways to manipulate it. B&H’s Kelly Mena does a great job at explaining it:
Basically, depth of field describes the distance between the nearest and farthest objects in an image, and how much in focus each one is. Calling depth of field “deep” means that most, or all, of the photograph will be in equal focus, whereas “shallow” depth of field means only the foreground will be in focus.
Generally portrait shots, like the one sampled below, look better with a shallow depth of field, because they force your eye to focus on the person.
The quality of the background, when blurred, is called bokeh. Bokeh is determined by the shape of the aperture blades and the focal length of your lens. Certain upper-class lenses are renowned for having sharper bokeh than others.
There are three ways to change depth of field:
Your aperture, or f-stop, is the most popular method of adjusting depth of field. The wider the aperture, the shallower the depth of field; if it’s narrower, your depth of field will be deeper, meaning more of it will be in focus.
For example, as seen above, an f-stop of f/16 is considered a very narrow aperture, so the background cars are somewhat clear. But when you get to the widest aperture, f/1.4, like what most prime lenses can achieve, you get a foggy background, focusing instead on the foreground exclusively.
Next up is focal length. A longer focal length creates a shallower depth of field. So if you zoom in with a strong lens, as in the image above, the closer you zoom into the object–even if it’s with the same aperture setting–will blur the background out more.
Lastly, you can move yourself in lieu of your lens to alter your depth of field. This is especially useful with prime lenses, a.k.a. lenses that don’t zoom. They’re excellent for portraits, but require you be fairly close to your subject. Remember: the closer you are, the shallower your depth of field will be. It’s the same premise as using a zoom lens.
There’s no right or wrong when it comes to depth of field, just what you prefer, and what kind of atmosphere you want to establish. And, of course, consider every aspect of photographer when toying with a new one.
“Don’t forget about composition. Make sure that whatever you’re shooting, you think about composition in combination with depth of field. Try not to sacrifice one for the other.” – Kelly Mena