Enewsletter
May 2010

Knowing about depth of field and how to use it is crucial information for photographers because it directly affects what your pictures will look like and whether objects at specific distances from the camera will be critically sharp, very blurry, or something in between. In short, controlling depth of field to express your intentions is a significant part of the creative process of photography. For example, if you’re shooting a landscape with a picturesque mountain range in the background and a weathered old wagon wheel located in the extreme foreground only 5 feet from the camera, you will need a very wide (extended) depth-of-field range to capture both elements with full detail. On the other hand, if you’re shooting a portrait, especially one with a cluttered background, you probably want to draw attention to your subject’s face and soften the background by setting your camera to provide a very shallow depth of field. Indeed, shooting portraits with extremely shallow depth of field so the subject “pops” off the background in a visually compelling way is a common technique used by many professional portrait photographers.

There are four elements that determine the depth of field in your pictures—the aperture or f/stop at which they were shot, the focal length spec or focal-length setting of the lens in use, the distance of main subject or particular objects from the camera (as measured from the image plane), and the degree of enlargement of the final image in a print or on the screen. Because the depth of field zone is shallower at closer distances and deeper at longer distances, the general rule of thumb is that depth of field at any given focusing distance extends for half of its total distance in front of the focusing point and twice the distance behind it—in other words 1/3 of the depth-of field zone extends in front of the object you’re focusing on and 2/3 extends behind the subject at any given aperture. This is a good approximation for normal shooting distances in the 6 to 30-foot range, but at focusing distances of 3 feet or less the depth of field in front of and behind the object is more nearly equal.

The most important facts to remember about depth of field is that for any specified format (full-frame 24x36mm or APS-C with most DSLRs), the wider the aperture (lower the f/number) and the longer the focal length (greater the focal-length number in millimeters) of the lens in use, the shallower the depth of field will be at any given object (camera-to-subject) distance. Remember that with prime (non-zoom) lenses the focal-length in use is the one engraved on the lens identification ring; with zoom lenses it’s focal-length setting used to take the picture.

Both aperture and focal length also affect depth of field independently of each other, so the wider (lower f/number) the aperture you use at any given focal length, the shallower the depth-of field zone will be, and the longer the focal length you use at any given aperture setting, the shallower the depth of field will be. It logically follows that if you want the minimum possible depth of field (narrowest sharpness zone) in your captured images for some creative reason, you might use a long telephoto lens (say, a 200mm, 300mm, or a long-range zoom) and shoot at a wide aperture such as f/2.8 or f/4. Conversely, if you require maximum depth of field, with everything from the closest to the farthest objects in the scene appearing tack sharp, you should shoot at a small (numerically large) aperture such as f/16 or f/22, and favor short focal length lenses that provide wide- or ultra- wide-angle coverage—10-16mm for APSC-format cameras, 15-24mm for full-frame 35mm cameras and DSLRs.

In the real world, most people don't decide which lens to mount on their DSLRs based solely on its depth-of-field characteristics—lenses are generally chosen on the basis of angular coverage (wide, normal, tele) or zoom range, that is, which lens best suits the subject and how you want to frame it. However, no matter what lens you’re shooting with or focal length you’re shooting at, the aperture dial will always let you control depth of field to some degree. To do so, you’ve got to get in the habit of taking your camera off P (program mode) or green mode (basic Auto setting) and switching to A (aperture priority) or M (manual exposure) mode which lets you select your shooting aperture directly. Many pros leave their cameras in A mode, pick the aperture they want, let the camera select the proper-exposure shutter speed, and fine-tune their exposures if need be using the plus/minus exposure compensation control. This is a great technique for digital shooting because you can assess the results instantly on the LCD.

For checking depth of field accurately and precisely, there’s really no substitute for evaluating the full image on a 17-inch or larger computer monitor, or by making large prints. However, you can get a handle on the depth-of-field characteristics of pictures you’ve taken (albeit less conveniently) by examining the images on your camera’s LCD by zooming in using the image magnification feature, and then moving the image laterally and vertically to assess critical areas. When you do any of these things it soon becomes crystal clear that the effective depth of field in any image is inversely proportional to the degree of magnification. In other words, the more any given image is magnified, the (shallower) narrower the visual depth-of-field range becomes. In an 8x10 print the flower held in your model’s hand and the wallpaper pattern in the background may both look quite sharp, but when you crop in a bit to make a 13x19 or 20x24-inch print of the same image, or view it on a really big screen, you may discover that these details look quite soft and blurry. Indeed, you may find that only her eyes remain needle sharp, and that the tip of her nose and ears are just barely in focus.

Shallow depth of field can be very attractive in portraiture, particularly portraits of women, girls, and children, but it’s generally not the hot ticket for product shots, cataloguing your antique knick-knack collection, or architectural studies. Anytime you need an extended depth of field range, with whatever lens you’re using, by all means stop the aperture down to f/16 or f/22. If the resulting shutter speed is too slow for handheld shooting, use a tripod, turn on the flash, increase the ambient light level by turning on more lights, or use a higher ISO (many current DSLRs perform splendidly in the ISO 800-1600 range.) Also, don't forget to take some shots at those mid-range apertures like f/5.6, f/8 and f/11. Many lenses are at their sharpest when stopped down 2-3 stops from the maximum aperture, and the moderate depth of field you get at these apertures looks very natural with many subjects, including scenic views and landscapes.

Theoretically, with an SLR you should be able to observe the depth of field directly in the viewfinder before you take the shot by stopping the lens down to its shooting aperture, but this isn’t possible with most modern DSLRs and it only gives a rough approximation in any case. To address this and other problems, lens and camera makers came up with the depth-of-field scale, a dual aperture scale symmetrically arrayed around the focusing mark (index) adjacent to the (movable) focusing scale. Some prime (fixed focal length) and macro lenses still have this useful device. Here’s how to use a depth-of-field scale if you are lucky enough to have a lens with this feature:

1. To find the depth of field range for any given focusing distance and aperture, set your focusing distance (say, 10 feet) opposite the focusing mark and read the near and far distance limits appearing directly opposite the same two aperture numbers (say both f/11s) on the adjacent depth-of-field scale—one is to the left of the focusing index mark, the other to the right. In this example (depending on the focal length of the lens) the near distance may be 5 feet and the far distance may be 20 feet, a total depth-of-field range of 15 feet. What determines whether the near-distance number appears to the right or the left of the index mark depends on whether the lens focuses closer by turning the focusing ring to the left or right.

2. To determine the minimum distance setting that provides depth of field all the way to infinity at any given shooting aperture, move the infinity mark on the focusing scale until it is opposite the closest aperture number on the depth-of-field scale that corresponds with your chosen shooting aperture (say, f/8). The focusing distance now appearing directly opposite the focusing mark or index is called the hyperfocal distance—the distance setting that provides the maximum depth of field at your shooting aperture when objects at photographic infinity must be sharply captured. Shooting at any distance setting farther than the hyperfocal distance is not generally useful—it may cause objects at close distances to be less sharp, but will not increase sharpness of distant subjects.

The final depth-of-field variable, and a very important one, is the shooting distance—the distance from the focal plane of the camera to the main subject. The farther the shooting distance, the greater the depth of field range (in feet) with any lens at any given aperture, and the closer the shooting distance, the shallower (narrower) the depth-of-field range will be. For example, a lens like a 300mm f/2.8 (long telephoto focal length, wide aperture) will provide a very shallow depth of field when the subject is at 10 feet and you shoot with the lens wide open at f/2.8—objects at 9 feet or 11 feet will probably appear quite soft and blurry. However the depth-of-field range will increase considerably, even at f/2.8, if you focus on your main subject at 40 feet. In this case, objects from 38 to 44 feet will probably look quite sharp. When shooting at very close distances (magnifications from 1:2 to 1:1) with macro lenses like the Tamron 90mm f/2.8, the total depth of field at f/2.8 may well be measured in millimeters, and even stopping down to a small aperture like f/16 or f/22 will only extend it to an inch or so in front of and behind the point of focus. The takeaway here: No matter what lens or aperture you shoot at, depth of field diminishes dramatically as the camera-to-subject distance diminishes, and below 2-3 feet or so, critical focusing is essential. Now you know why many prime macro lenses provide very small apertures like f/22 and f/32 to maximize the very limited depth of field in objects shot at ultra-close distances.

To review, let’s distill the main depth-of-field facts you need to remember as a list of convenient bullet points:

1. The longer the effective (marked or set) focal length of the lens you’re using, the shallower (narrower) the depth of field will be at any given aperture or focusing (shooting) distance.

2. The wider the shooting aperture (lower the f/number) the shallower (narrower) the depth of field will be at any given focal length or shooting distance.

3. The greater the magnification of the captured image (degree of enlargement in a print or on the screen) the shallower (narrower) the depth of field will appear so long as the image is viewed at a standard viewing distance (i.e. you don't back off to view the entire image.)

4. The greater the object distance (the distance from the imager plane of the camera to the main subject), the greater the depth-of-field will be with any particular lens and at any shooting aperture.

Keep these facts in mind whenever you take pictures, and be prepared to set your camera to A (aperture-priority) or M (manual) mode a lot more often and you’ll be well on your way to mastering depth of field, one of the most crucial creative elements in all of photography.