Also called DOF, depth of field is the distance between the nearest and farthest objects in the frame that appear in focus. When a large depth of field is used, it can produce images like this:
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When a short depth of field is used, it produces images that look more like this:
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Depth of Field is commonly used on portraits, which is a picture of someone's face, like this (to keep their face in focus, with the background smoother):
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It is also common for things to be close-up when having a short depth of field, like so (which can also lead to bokeh which are the out-of-focus dots that make up blurry shapes):
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When used in film, depth of field can be altered while shooting, which can change the focus from one plane to another or simply change to include more or less space in focus. This can help direct the focus of the audience's eye as well as create interest by blurring some parts of the frame and then revealing them in focus.
This technique can be difficult to utilize with less advanced cameras that do not have the option to manually focus. In the upcoming project, one might use a shallow depth of field shot to open a sequence (a close shot of your athlete in the gym with the background blurred, a nutritious meal with just one element in focus, etc).