Thursday, November 22, 2012

What Is Focal Length?

 The explanation that a lens’s focal length is the distance from the optical center of the lens to its point of focus doesn’t tell the whole story, of course. What is important is that the focal length determines the relative field of view produced by a lens how wide or narrow its perspective is. A lens with a wide perspective will provide an expansive view like the one shown in Figure 1.10. One with an intermediate perspective (which photographers call normal) offers a view like that in Figure 1.11. A narrow, telephoto view might bring details of a subject in very close, as shown in Figure 1.12.

Focal lengths are measured in millimeters. Some lenses have a field of view that is fixed at a particular focal length, such as 18mm, 50mm, 105mm, or 200mm. These are called prime lenses or, sometimes, fixed focal length lenses. Other lenses have the ability to shift lens elements around to produce a continuous range of focal lengths. These are called zoom lenses. A typical zoom lens might be able to change magnifications from an 18mm wide view to a 200mm telephoto perspective, thus incorporating the fields of view of the 18mm, 50mm, 105mm, and 200mm prime lenses listed previously, plus all the focal lengths between them. Whether a given focal length is considered wide, normal, or telephoto depends on the size of the sensor (or film) used to capture the image. For that reason, a lens that might be considered wide when used with one digital camera, might be categorized as a normal lens when used with a digital camera that has a smaller sensor. Because most consumer digital SLRs have sensors that are similar in size, the most common focal lengths can be grouped into general categories, with, of course, some overlap between adjacent groups. The following list shows the approximate focal lengths of lenses used with cameras that have a 1.5x or 1.6x “crop factor,” with a little overlap in focal lengths between some categories (because that overlap exists in real life). But remember that if you are comparing these focal lengths with lenses used with film cameras or some professional digital cameras with no crop factor, and other models with a different crop factor, the guidelines won’t apply.
 
Ultra-Wide Angle: 10-15mm. : These lenses provide the broadest view, taking in large swathes in landscape photos, or virtually all of an interior space. Objects closer to the camera may appear larger and the foreground is emphasized, as you can see in Figure 1.10.
Wide Angle: 16-28mm. : Use these lenses for most landscape, architecture, and interior photography, or any subjects where you have a wide field of view.
Normal: 28-40mm. : This focal length range is defined separately because, before zoom lenses became predominant, most cameras were purchased with a “normal” lens (a full-frame focal length of 50mm), with an unremarkable field of view that was neither fish nor fowl/wide nor telephoto. While the normal range gained a reputation as “boring,” prime lenses of this focal length have large maximum apertures (often f/2 to f/1.4), which makes them ideal for low-light photography. Normal focal lengths are also good for 3/4 and full-length portraits, or shots of small groups.
Short Telephoto: 40-60mm. :  Lenses in this focal length range have been traditionally called “portrait” lenses, because they provide a flattering perspective for headand- shoulders (and closer) portrait images. Focal lengths shorter than this range can exaggerate the size of features like noses that are closer to the camera, at the expense of features like ears, which are farther away and appear to be too small. Focal lengths of about 100mm or longer tend to compress facial features together in a flattening effect.
Medium Telephoto: 60- 135mm. : Lenses in this range (as well as those at the long end of the short telephoto range) are popular for close-up and macro photography, because they let you maintain a little distance between your subject and the camera. That distance makes lighting the subject easier, and can be less threatening to small living subjects, such as insects. Medium telephoto lenses are also useful for sports at close range, and some portraits, at the
shorter (61-90mm) end of the range.
Long Telephoto: 135-300mm. :  These focal lengths are useful for pulling in any subject that’s too far from the camera to fill the frame. You’ll find them helpful for shooting concerts, sports events, and skittish wildlife. longer telephoto lenses require solid technique to minimize the effects of camera shake (image stabilization, a tripod, and/or faster shutter speeds are required) and to manage the reduced range of sharpness these lenses offer.
Super Telephoto: 300mm and above. : Really long focal lengths are most useful to wildlife photographers, who must photograph creatures from hundreds of feet away (or smaller critters from dozens of feet away). Outdoor sports photographers who want to put themselves into the middle of the huddle, at the edges of the scrum, or capture an exciting play from the other side of the stadium or field also benefit from really long focal lengths. It’s true that 300mm just barely qualifies for this category: the really long focal lengths range from 400 to 600mm. Anything above 600mm is likely to be an exotic set of optics with an exotic price, as well.

1 comment:

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