How-To: Understanding & Applying Depth of Field
How-To: Understanding & Applying Depth of Field
Text and Photos by Lynne Eodice
Depth of field is the zone in your photograph that’s in sharp focus in front of and behind your main subject, and which has a profound effect on the way your images look. Here are two essential terms to know:
• Shallow depth of field — Characterized by a blurred background and/or foreground.
• Great depth of field — Denoted by overall image sharpness.
Three factors that determine DOF
Shallow DOF vs Great DOF: Cherry blossoms at f/4.5 and f/29
Click images to enlarge
First of all, your camera’s aperture setting plays a critical role in depth of field. The smaller the aperture setting, the greater the depth of field. Although it may seem somewhat counter-intuitive, a small aperture is a large f-number on your aperture dial (such as f/8, f/11, f/16, f/22 and so on), while a wide aperture is a small f-number such as f/2.8, f/3.5, f/4.5 or f/5.6). It’s easier to grasp this concept if you think of these numbers as being part of a pie: A fourth of a pie (f/4) is much larger than a sixteenth of a pie (f/16).
At f/16, most “normal” focal-length lenses (in the 35mm to 40mm range for most digital cameras) that are focused on a subject 12 feet away will render everything sharp from about 5.5 feet in front of the camera to infinity. At f/2.8, only the subject will be sharp; both the foreground and background will be blurred. Midway in-between at f/5.6, you’ll only get sharpness from about 3.5 feet in front of the subject to eight feet beyond it.
Secondly, subject distance affects depth of field. Generally, the closer your subject, the shallower the depth of field. Even at f/16, if you focus on a subject that’s three feet away with a lens in the normal range, the depth of field will be less than a foot. At f/2.8, your subject’s eyes may be in focus while the nose and ears are blurred. Conversely, when you back away from a subject the depth of field increases. With an aperture of f/16 at six feet, the zone of sharpness will extend from about a foot in front of the subject to about three feet behind it.
And finally, the focal length of your lens plays a role in your depth of field. The more magnification (the longer the focal length of the lens), the less depth of field you’ll have at any distance and aperture. Conversely, you’ll get greater depth of field with a wide-angle focal length at any given aperture. So if you’re shooting a scene at an aperture of f/8, for example, you’ll get greater depth of field when using a 16mm focal length than you would with a 35mm focal length, which would give you greater depth of field than a 100mm focal length.
Choosing Between Shallow and Great DOF
Keeping these three depth of field factors of in mind, you should select your lens focal length, distance from the subject, and aperture carefully. When the depth of field is what you want to control (as opposed to motion effects, which are controlled by your camera’s Shutter Priority mode), you should use your camera’s Aperture Priority mode (often abbreviated Av on the shooting modes dial). Here are some general rules of thumb for deciding when to select shallow or great depth of field:
• Use a large aperture and a telephoto focal length for shallow depth of field. Use a wide aperture when photographing portrait and close-up subjects, or any time you want to separate your subject from its background. The apertures between f/1.4 (or whatever your maximum aperture is) and about f/5.6 will isolate your subject from its background, particularly when you use a focal length of at least 40mm with most digital cameras.
• Use a small aperture and a wide-angle focal length for great depth of field. Use a small aperture when shooting landscapes, travel scenes, architecture, or whenever you want to show as much detail as possible. You can often get great depth of field with apertures between f/8 and f/11, especially when you’re using a wide-angle lens in the range of about 10mm to 16mm. However, when overall sharpness is critical, use apertures between f/11 and f/32.
Just remember that these guidelines are just generalities. For example, there may be times when you’ll want to shoot pictures of people with a small aperture when you want both your subject and the scenic surroundings to be in focus. Or you may want to zoom in on an interesting seagull with a wide aperture and allow the bay in the background to be blurred.
Controlling DOF with the Basic Scene Modes
You can control aperture and shutter settings with practically all digital cameras that accommodate interchangeable lenses, and even with many compact cameras that have built-in lenses. But if you own a point-and-shoot model that doesn’t allow you to control f-stops and shutter speeds, you can utilize your camera’s basic scene modes. For instance, if you want to shoot a portrait and have relatively shallow depth of field, you can use your camera’s Portrait mode (often denoted with a symbol of a woman wearing a hat). Your camera’s Close-up mode (the flower symbol on the shooting mode dial) will allow you to get a little closer to your subject and will limit depth of field as well. For times when you want to get everything in focus near and far, you can select the landscape mode (the mountain symbol on the shooting dial). Today’s compact cameras have a variety of shooting modes too numerous to mention here (as well as most DSLRs). The most important thing is to experiment with your camera’s settings and be creative!