In Focus: Understanding Depth of Field
September 2, 2014
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Depth of field is an important aspect of photography that professionals use to create great photos. Knowing how to get just the right amount of the image in focus can be the difference between a new photo in your portfolio and just another cluttered shot on the cutting room floor. Understanding depth of field means knowing the mechanics behind it as well as how to apply it properly.
Depth of Field Overview
Depth of field in a photograph refers to the range of sharpness in front of and behind the point on which the lens is focused. Only subjects at the exact distance at which the lens is focused are technically "in focus," however the human eye isn't particularly good at picking up on very small amounts of unsharpness, hence depth of field is a range and not a single fixed number. Depending on several factors, the depth of field of a photograph can be very shallow, with only a tiny sliver of the image acceptably sharp; very deep, with much of the image acceptably sharp; or somewhere in between.
Factors That Affect Depth of Field
Aperture: In addition to controlling the amount of light entering the lens, aperture also affects depth of field. Larger apertures yield a shallow depth of field, smaller apertures a deeper depth of field. There is a limit to how much a lens can be stopped down to achieve a deeper depth of field however, as diffraction increases and overall image sharpness begins to drop.
Focal length: The focal length of the lens plays a role in depth of field, too. Longer focal lengths make it easier to obtain shallow depth of field compared to shorter focal lengths at a given aperture. While mathematically the actual depth of field is virtually unchanged with focal length, the magnification of longer focal lengths does make the depth of field appear shallower.
Distance from subject: If all other factors remain constant, getting closer to the subject will give you a more shallow depth of field, while moving further away will give you a deeper depth of field.
Sensor size: This factor is the least easily changed. The only way you will notice any difference here is if you have more than one camera with different sensor sizes, but it's still worth mentioning. If all other factors remain constant, the larger of two sensors will have the more shallow depth of field.
Shallow Depth of Field
Having a narrow slice of the image in focus is helpful when you want to emphasize or isolate your subject as much as possible. Portraits, close-up detail shots, and macro photography are just a couple of instances in which shallow depth of field is a good composition strategy. When employing a shallow depth of field, don't neglect to consider the bokeh of your lens and how the negative space interacts with the subject. Balance an interesting use of negative space without distracting from the subject.
Deep Depth of Field
Sometimes you need to have more of the image in focus to get your point across. Landscape, architecture, and home and office interior photography are a few examples of when you might want to use a deeper depth of field to showcase as much of the scene as possible. It's important to remember that the smaller apertures required will result in slower shutter speeds so a tripod may come in handy. You also have to be hyper aware of everything in the frame — that forgotten soda can in the corner of the kitchen you're shooting could be a real distraction if it's perfectly sharp.
Choosing the right depth of field for a particular photo is important — it can help your subject stand out from the background, or it can show more of the environment your subject is in. As with all aspects of photography, practice makes perfect, so get out there and start experimenting!
Image source: Flickr