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Understanding depth of field and taking control of its effect on an image is an important skill in photography. Depth of field describes the area of acceptable focus in an image. Even though a camera is focused on a specific point in a scene there is a range of space in front of and behind that point that appears to be in focus as well. This range of focused space can be controlled by the photographer and used for creative effect.
A classic example of controlling depth of field can be seen in portrait photography. When a photographer takes a portrait the goal of the image is to focus attention on the subject. By controlling the depth of field the person’s face is sharply focused while the background is a soft blur.
While there are several variables that help create shallow depth of field in a portrait, in combination the outcome is an image that both isolates the subject from the background and creates a feeling of dimension in a two dimensional picture. Photographers can to a great degree control what is and what isn’t in focus in the final image.
The variables that photographers use as tools in creating and controlling depth of field are: lens selection, aperture and magnification. Shorter length lenses set for smaller apertures at a greater distance will have increased depth of field. Longer lenses at wider apertures at closer distances will have decreased depth of field.
Turning first to lens aperture the rule of thumb is that larger (wider, faster) apertures create shallower depth of field. Here is a scene to consider: the photographer is shooting a subject that is six feet in front of a bush or tree. If the camera is set to use an aperture of f/8.0 both the subject and the background will be in focus. If instead the photographer sets an aperture of f/3.5 the subject remains in focus but the background is thrown into a blur.
Lens selection impacts depth of field too. The longer the lens focal length is the more shallow the depth of field will be. A 28-75mm zoom lens set to 28mm will offer great depth of field while the same lens set to 75mm will offer much less. This has to do with several technical issues such as hyper-focal length and image compression that we won’t get into here. However we can easily prove this effect by recreating our test scene and then shooting two pictures without moving the camera; one image at our shortest focal length and another at the lens’ longest length.
Last we have magnification. Photographers are used to thinking of magnification in terms of lens length but here we mean how close or far the camera is from the subject. An image taken with the camera closer to the subject will have less depth of field than an image taken when the camera is moved farther away. (Using the same lens length at both distances.)
We can get a sense of how these three tools are applied in general terms by looking at three types of photography:
- The landscape photographer will lean toward short length lenses while using small apertures. In general their subject is at a considerable distance too. The result is an image with tremendous depth of field. Everything from their toes to the horizon seems to be in focus.
- On the other hand the portrait photographer looks to isolate their subject from the environment. By using a moderate telephoto lens (75mm to 105mm) set to a wider aperture and shooting from a distance of six to ten feet the resulting image will have a sharp subject and a blurred background. The degree of blur is controlled by the aperture most often.
- The macro photographer by necessity is shooting with the camera positioned very close to the subject (high magnification). In addition most macro photography is done with a longer lens length. The longer lens permits lighting the subject without casting the camera’s shadow on it. However a camera placed extremely close to the subject and with a long lens mounted is a recipe for unacceptably shallow depth of field – perhaps measured in millimeters. For this reason the macro photographer will set the lens aperture to the smallest extremes in order to maximize the depth of field, favoring f/22 and smaller if available.
Of the three tools available for depth of field control the lens aperture setting is the most measurable and repeatable. For this reason the majority of intermediate to advanced photographers will have their cameras set to Aperture Priority mode (Av) if it isn’t in full Manual mode. Program mode will occasionally hit a setting that creates pleasing depth of field but it’s more often just a happy accident when it does.
Exercising control over depth of field is as much a part of good photography as exposure or composition. Using it to best advantage is a skill worth developing. I suggest practicing using the tools discussed in this article and spend some time reviewing the images you capture in large size on your computer monitor and set the view to show the camera settings with the image.
Be somewhat systematic in learning to use depth of field by taking the same image several times varying one tool only: lens length, aperture or magnification. Keep in mind the relationship between the three tools:
Wider lens + smaller aperture + greater camera to subject distance = greater depth of field
Longer lens + wider aperture + closer camera to subject distance = shallower depth of field