Intro to Depth of Field

Minolta XD-11, MD Rokkor-X 50mm 1.4, Vivitar MC 2x teleconverter
Kodak BW400CN

Simply, depth of field (DoF) defines how much of the scene is in focus — both in front of and behind the focus distance. It’s another composition tool available to photographers, but it isn’t incredibly intuitive to pick up, so I’ll touch on the largest determining factors.

The aperture consists of multiple blades which can be closed down to limit the amount of light into the camera (like an eye’s iris). The aperture is the easiest control of depth of field, and the aperture is rated by f-numbers (which is a ratio of the focal length over the aperture diameter). So, the smaller the the f-number, the larger the aperture. For example, a f/2 is larger than a f/4.

The effect of opening or closing the aperture has two side-effects. The most obvious is the amount of light that reaches the film/sensor. For example, one of my lenses can close down 7 stops: f/1.4 is ~128 times brighter than f/16.

Secondly, depth of field is directly affected by the aperture such that a higher f-number results in a larger DoF. Effectively, the smaller the aperture is set, the more that will be in focus. The converse is thus true that a shallow DoF is the result of a larger aperture (lower f-number).

DoF is affected by various other factors that I’ll merely mention. For instance, wide angle lenses typically have greater DoF than standard. Also, the closer you’re focused towards infinity, the larger the DoF. I’ll revisit these ideas again later in an article about understanding and using hyperfocal distance.

Anyway, time for examples:
Ooh... Pretty.
Minolta XD-11, 50mm MD Rokkor-X f1.4, on Fuji Superia 400.

A shallow DoF can be used to isolate the subject from the background for whatever reason — distracting, ugly, etc. In this case, with a large aperture and close focal distance, the trees/yard/road are blurred into oblivion to create a dream-like state.

Yashica Electro 35 GSN, Fuji Superia 200

Controlling DoF can be incredibly useful in portraiture. Here, background blur is present, but not overwhelming. Had the background been sharper, it could become distracting; more blurry, then less interesting.

Minolta XD-11, 28mm MC W.Rokkor-X f2.5, on Kodak Tmax 100.

This is one of my favorites, and it’s a good demonstration that background blur (shallow DoF) is absolutely not necessary for a great photo. There is no “right way” to use DoF — it, like everything else in photography, is merely a tool. No more, no less.

(Not all cameras have access to aperture controls. Some point and shoot digital cameras allow it, but camera/lens design often limits the flexibility of such cameras. Consult camera documentation to see about access to either a manual or aperture priority mode).


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