Depth-of-Field—It can either work for you or against you . . . if you do not fully comprehend what it does and how to control it. So, why leave such vagaries to chance? Today we’ll discuss ways to make depth-of-field work to your advantage.
Maximum depth-of-field will present objects in both the foreground and background in the sharpest possible focus. Minimum depth-of-field is used to keep a single subject in sharp focus while deemphasizing the distractions by blurring anything in the foreground or background that is not germane to the subject.
Landscapes—Here you generally want maximum depth-of-field, allowing you to capture in sharpest possible detail everything from middle distances out to the horizon.
Portraits—Generally speaking, these call for a very narrow depth-of-field. You want to have your subject in sharp focus, but if the background isn’t blurred then it will compete for the viewer’s attention. This may not be the case if you want your subject framed against the backdrop of a noted landmark such as the Eiffel Tower, or with the Grand Canyon residing in the background. In those cases, your “portrait” shot will need to be more like a landscape.
Still Life Photography—If your subject is a single flower, think of your photograph as a “portrait” of that flower and blur the foreground and background accordingly. If your subject is a collection of brightly colored fruit, a large bouquet or a bed of flowers, then maximum depth-of-field may be required to bring out the highest degree of visual impact. But in this latter case, you may still want to find a depth-of-field middle ground to blur background distractions.
Controlling Depth-of-Field (DoF)—There are many ways to control this aspect of photography. Generally speaking wide-angle photography has more DoF than telephoto settings; smaller camera sensors give greater DoF than do larger sensors at the same lens aperture settings and relative focal lengths (think pocket camera versus DSLR here); and wider apertures narrow the DoF while smaller aperture settings increase the DoF.
Inherent to the above paragraph is the understanding that you’re going to have a tougher time reducing your DoF with a small camera than you would if you have a camera with a larger imaging sensor. On the other hand, this means that a point-and-shoot will have a built-in advantage over your DSLR when it comes to providing maximum depths-of-field. Also inferred is that if you own a DSLR and do a lot of portrait-style or other narrow-focus photography, you need to consider investing in more expensive, wider aperture zoom lenses, or perhaps even go to fixed focal length (non-zoom) lenses for some of your photographic work as fixed focal length lenses are often available in wider apertures.
The best focal lengths for portraits, whether you use a small pocket camera or a larger DSLR, are (in 35mm equivalent) in the 85mm to 135mm range, with 105mm being my personal “sweet spot” for optimum portrait DoF control. Indeed, my favorite all-round zoom lens for everything from landscapes to portraits is a 24-105mm f/4.0 mounted to a DSLR with a full-size (35mm) sensor. Professional photographers will go with dedicated 85mm and 105mm f/2.8 fixed focal length lenses, but I personally just can’t justify that kind of expense for a hobby.
But watch out for the extremes—You may be tempted to go with the maximum aperture available for portraits and the smallest possible aperture for all your landscapes. Resist the temptation unless you really need it. The widest aperture will frequently give you vignetting (light drop-off) at the corners of your photograph, and most lenses also produce a softer image when set this way (not necessarily a bad thing in true portraits, as softening helps to hide skin imperfections and blemishes). At the other end of the aperture spectrum many lenses are prone to chromatic aberration (also known as “color fringing”), which causes different colors to focus at differing points in the photograph. This can result in an odd, distracting, rainbow-like effect at the edge of a subject that diminishes clarity and sharpness. Another problem associated with DSLRs set to minimum aperture is dust and dirt on the sensor. These contaminants (which may otherwise be rendered all but invisible at wider apertures) will come into sharp focus at f16 to f22, especially against a uniform background such as blue sky. Sure, you can edit these specs out later in post-processing, but why make your life more difficult?
Now, a little trick for even greater Depth-of-Field—
The Setup: You have a potentially award-winning composition. In the foreground lies a field of amber grasses with long, flowing stalks. The background displays an impressive range of majestic, snow-covered peaks against cerulean skies. A warmish, setting sun is directly behind you, giving both the grasses and the mountains a pleasing, surreal reddish hue.
The Problem: Even at the narrowest aperture your lens allows, if you focus on the grassy foreground, the mountains lose their crisp focus. If you lock on the mountains, those amber waves of grain become a blurry, confusing mush.
The Solution (for DSLRs and higher-end point-and-shoots): Take your camera off autofocus and manually focus at a point between the foreground and background, with the focusing point biased toward the foreground. Optimally, this focal area will be at an imaginary point one third the distance from the closest foreground subject, two-thirds away from the farthest object.
The Solution (for “consumer” point-and-shoots): If your camera does not allow for manual focusing but does let you select from one of several available focusing points, select a focus point between the foreground and background biased in favor of the foreground as described in the above solution for DSLRs. If your camera does not allow selective focus points, focus directly on a portion of the scene as described above—one third of the distance past the foreground and two-thirds the distance in from the background—by depressing the shutter release button halfway. Continue this halfway press to lock in the focus (which will also lock in the exposure settings, by the way), recompose the scene, and then depress the shutter release button the rest of the way to snap the shot.
The following photographs were taken using a Canon EOS 5D with a 24-105mm f/4L IS lens. Focal length and aperture settings are as listed. Subsequent shots in each sequence represent a full stop decrease (halving of the amount of light falling on the sensor) of the preceding shot. Thus, the apertures in each sequence are: f4.0, f5.6, f8, f11, f16, and f22 (click on this link for my previous discussion on f-stops).
Note how the DoF increases as the aperture decreases (remember that when discussing aperture in photograph, the higher the number the smaller the lens “opening,” or “aperture”). Also, you can see examples of the aforementioned vignetting problem in the f4 photographs of in the 60mm and 105mm series—look at the darkening of the blue sky in the upper corners.