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|Single Image Max DOF. Narrowing the lens aperture maximizes depth of field, as in this example of a field of flowers, rendered sharp from edge to edge and front to back with a 17-40mm lens at 17mm and ƒ/16. Canon EOS-1N film camera and Kodak Ektachrome E100S film.|
Sharpness is the most essential technical element of composition and quality in most photographs. We're obsessed with maximizing the range of sharpness (aka depth of field) in our images—achieving it, and positioning it to enhance the message, is evidence of our skills as photographers and artists, and arguably the first criterion by which our work will be judged. Whether landscape, wildlife, sports or macro specialists, outdoor and nature photographers are pretty much always in search of sharpness. To maximize sharpness, a thorough understanding of depth of field is helpful—what it is, and how to expand it and precisely apply it in your photography, from film and digital single captures to advanced digital multiple-image composites.
Depth Of Field 101
Depth of field (DOF) is the distance within an image between the nearest and farthest objects that appear acceptably sharp. Note that for any combination of sensor/film, lens, focal length and ƒ-stop setting, there's only one precise point of focus; the rest of the area attributed to DOF is technically unsharp but perceived to be acceptable, with the sharpness decreasing as the distance from the precise plane of focus increases. In the defocused areas, the spreading diameter of each pinpoint of information is called the "circle of confusion." An acceptable maximum numeric size for this point, or dot, in 35mm images is said to be 0.025mm (0.001 inches), and some DOF charts use an acceptable circle of confusion of 0.030mm for digital images.
Keep in mind that the area of sharpness in the foreground and background transitions gradually from sharp to out of focus. Thus, defining the acceptable area of focus—the DOF—depends also on our eyesight, the size of the image we're viewing, the viewing conditions and whether the photographer has placed the DOF advantageously to the photograph's subject and message (its composition).
The range of DOF is directly related to the size of the film or sensor being used. If we keep the subject size the same, maintain the same distance to the subject by modifying the focal length and keep the same ƒ-stop, the smaller format will have more DOF because the larger formats will need a longer focal length to match the size of the subject in the image. You see this when using a compact digital camera (point-and-shoot) with a 1⁄2-inch sensor at ƒ/5.6. You have a ton of DOF because the focal length of the lens used is approximately 5mm to 25mm. The 5mm focal length is equivalent to a 24mm lens on a 35mm (full-frame digital) camera. As a side note, this is the reason why full-frame and APS-C DSLRs with video have become so popular for motion shooters; the DOF is comparatively very shallow, allowing precise focus on the subject and throwing distracting backgrounds out of focus. A camcorder with a tiny sensor won't allow the shallow DOF that defines the cinema look.
As complex as it may sound, DOF isn't just an indelible law of optics that we're helpless to control; if we understand it, we can use DOF to maximum advantage in our photography.
Controlling Depth Of Field In Single Captures
On film and digital sensors, expanding DOF is a continuing challenge for photographers, but it can be achieved in single captures, with varying success, in several ways. Using a wide-angle lens (16-24mm for full-frame DSLRs) offers nearly unlimited DOF, but tends to create a distorted perspective. Moving farther away from the subject (say, an imposing tree in the middle ground of a landscape image) can bring the primary subject into focus; however, this isn't always desirable, possible or practical. On the other hand, using a telephoto lens at larger apertures is a good option for minimizing or strategically placing DOF. This "selective focus" can be a strong compositional tool.
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Stopping down the aperture of the optic is typically the best option to increase DOF. For all lenses, the smallest lens opening offers the greatest DOF. But be aware that at smaller apertures like ƒ/22, for example, your expanded DOF will be compromised by diffraction—the bending of light rays as they enter the very small lens opening at an angle. Furthermore, a smaller ƒ-stop typically mandates a longer shutter speed, which renders the image more susceptible to camera and subject movement. All of these variables must be considered when attempting to achieve the largest possible area of sharpness.
Another technique to maximize DOF is tilting the front optic to induce the Scheimpflug principle. Normally, the zone of focus is parallel to the aligned front lens and the film or sensor, but if the axis of the lens is tilted, the zone of focus tilts with the new axis. Imagine, for example, that you're photographing a field of flowers. Normally, the plane of focus is vertical, ranging from the ground, up the stems of the flowers and straight into the air above them. Tilting the plane of focus to skim across the tops of the flowers and into the distance applies the available DOF to the area of the image that's most important. Today's tilt/shift lenses work on this principle, and large-format view cameras have been using the front tilt since glass plates and sheet film were invented.
For each lens and ƒ-stop combination, there's a point—the hyperfocal distance—from which all objects will be sharp into infinity. The greater the focal length and the larger the aperture, the farther away the closest focus point will be. Charts are readily available to assist in these calculations. Some prime wide-angle lenses actually have DOF markings on the lens barrel to make the setting easier to attain. By placing the marking for a certain ƒ-stop on infinity and reading its corresponding marking on the focus indicator, you'll have set an approximate hyperfocal distance for these lenses.
A prime example of a useful hyperfocal distance calculation is a wide-angle lens of 16mm set to ƒ/16: the area from one foot in front of the lens to infinity will be sharp. The lens focus indicator must be set to two feet for this to work. Think flowers in the foreground and majestic mountains in the distance. To attain this maximum hyperfocal distance for a 105mm lens at ƒ/16 would require the photographer to set the lens focus indicator to 88 feet, and everything from 44 feet to infinity would be sharp. To take this to an extreme, my 500mm telephoto lens at ƒ/16 needs to be set at 1710 feet to get everything from 855 feet to infinity sharp. (Good luck in finding a setting for 1710 feet on the lens.) I carry a hyperfocal chart in my camera bag to assist in setting my focus point with various lenses and ƒ-stops. If you use an iPhone, check out the Focalc, DOFMaster or DoF Calculator apps; there are many more for Apple and Android phones. Just type in your camera, lens and ƒ-stop, and the app tells you the focus distance setting and what the nearest sharpness will be.
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Image-Stacking For Unlimited Depth Of Field
We keep saying that the digital age of photography has freed us from many of the limitations of film. With DOF, this is especially true; we now have the potential for unlimited sharpness because of the ability to composite large numbers of images. This process is called "stacking," taking a series of images, each at a different but overlapping focus zone, and processing the images in stacking software to output a finished composite image that retains only sharp areas. The resulting image can have unlimited or controlled DOF, depending upon the photographer's intent.
Stacking to improve DOF is useful for all subjects. I regularly use it in landscape compositions to achieve sharp focus from immediately in front of the camera to infinity. Stacking can be especially helpful when working with longer focal-length lenses, where DOF is naturally quite shallow. A limitation with stacking is that neither the subject nor the camera can move during each exposure, so it's not effective on a windy day or with, say, running horses in the composition.
Macro photography presents the most difficulty in attaining DOF, and stacking capabilities have revitalized my interest in high-magnification work. Consider that, at 1X (life-size), the DOF at ƒ/16 is only 2.24mm. At 5X, it's a mere 0.269mm. Not everything is perfect or easy in stacking macro images. Complete elimination of movement of the camera and subject is even more imperative at high magnification, and if a close detail obscures elements behind it in the composition, there can be a blooming effect that causes an out-of-focus halo in the foreground.
Making precise, small adjustments to focus is a big challenge. At lower magnifications, the focus can be changed in incremental, visually monitored movements of the lens focus ring, but as the magnification increases, either the camera/lens assembly or the subject must be moved at fractions of a millimeter. Enter the StackShot (www.cognisys-inc.com), a focusing rail with a step motor at the back that moves the camera in very small, precise adjustments (from 100mm to 1 micron) that are calculated and preset on a controller. Either a tripod or copy stand is necessary to hold the StackShot and camera assembly. A battery system is available for field work.
Basic Steps For Depth-Of-Field Control
|The two things we usually want to do with depth of field (DOF) are to maximize it or to place it strategically within an image. Work with these basic steps to become familiar with the ways your camera, lens and exposure choices affect DOF.
1 Choose the lens and focal length needed to properly frame the subject.
Once captured, the set of images must be composited in stacking software. The first software I used to apply the stacking technique was Helicon Focus, developed some years ago by Danylo Kozub in the Ukraine. The program has been improved regularly and can be found at www.heliconsoft.com. Recently, I discovered another stacking program, Zerene Stacker (www.zerenesystems.com), which I find to be very versatile and especially good for "deep stacking," that is, combining more than 100 images for DOF at high magnifications, from 5X to 10X. Adobe Photoshop CS4 to CS6 (www.adobe.com) also include a stacking feature within the Blend modes.
Shoot It Right
While the digital era gives photographers a whole new range of powerful postprocessing tools that can be used to improve images, depth of field isn't one of those problems that can be "fixed in Photoshop." As always, the capable photographer keeps postprocessing options in mind at capture and also seeks the best quality possible in the camera. The search for sharpness is such a basic element of photography that it's worth the effort to master and apply a full range of techniques.
See more of George Lepp's photography at his website, www.georgelepp.com.