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Tuesday, November 27, 2012

In Search Of Sharpness

How to take control of depth of field in-camera and in the computer

This Article Features Photo Zoom

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.

DOF Scale. A tilt/shift 45mm lens has a depth-of-field scale to assist in setting the hyperfocal distance on the lens. If the camera/lens aperture is set to ƒ/22, and the ƒ/22 marking on the scale is set to infinity, the near focus will be 5 feet and the center focus indicator will read at 10 feet. You'll find this scale only on prime lenses, usually wide-angles.
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).

Format Size. Sensor format size affects depth of field. 1) A capture at 100mm ƒ/4.5 on a full-frame digital camera is limited in depth of field, with only one flower in the array critically sharp. 2) Photographed from the same distance at 25mm ƒ/4.5 on a 1⁄2-inch sensor digital point-and-shoot, nearly all the flowers are rendered sharp. In each image, the subject was framed the same; that is, there was no cropping.
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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