Tuesday, February 25, 2014
Super Telephoto Zooms
We decipher the technobabble behind these nature photography mainstays
A long lens can "bring the subject to you," producing a larger image in the image frame and making long lenses particularly popular with wildlife and bird photographers. But they can also be useful to landscape shooters, as they allow you to zero in on more distant portions of a scene, and in so doing, flatten the perspective. (Note: It's the great distance, not the focal length, that compresses the perspective. If you shoot from the same spot with a shorter lens, then crop the resulting image to match the area shown in the longer-lens image, the perspective will be the same.)
Telephoto zooms offer additional benefits. First, as with any zoom, you get a whole range of focal lengths in a single package. That makes reframing much simpler (just rotate or push/pull the zoom ring, rather than physically change lenses) and minimizes dust on the image sensor by minimizing the number of lens changes in outdoor conditions. Telephoto zooms can take you out to 500mm for not much over $1,000, while a good 500mm prime lens can cost five to 10 times that. But a telephoto zoom offers another advantage over a single-focal-length lens. You can zoom back to the widest focal length to "find" your subject, which is convenient for wildlife and especially useful for birds in flight, then zoom in to frame, as desired. It can be hard to acquire a small or fast-moving subject with a long prime lens and its very narrow angle of view.
For our purposes here, "telephoto zoom" means one with its entire focal-length range in the longer-than-"normal" category: beginning at 70mm for full-frame cameras, at 50mm for APS-C cameras and at 35mm for Four Thirds DSLRs (Four Thirds System DSLRs are no longer in production, but some readers no doubt have one, and Four Thirds lenses can be used, via adapter, with Micro Four Thirds cameras).
What Does Telephoto Mean?
In general, lenses close in focal length to an image format's diagonal measurement are considered "normal" for that format. For example, a 35mm film frame (or a "full-frame" DSLR sensor) measures 36x24mm and has a diagonal measurement of 43.2mm. Lenses in the 40-55mm range are considered "normal" for this format.
Lenses shorter than the format's "normal" lens take in a wider angle of view and are called "wide-angles." Lenses longer than a format's normal lens take in a narrower angle of view, but instead of calling them "narrow-angle" lenses, most photographers (including us here at OP) tend to call them "telephotos." Actually, "telephoto" refers to a specific optical design in which the focal length is longer than the lens' physical length. But most of today's long lenses are indeed telephotos, which is kind of nice: It makes them less bulky. "Long-focus" is another term for these lenses.
Bottom line: A telephoto lens has a focal length longer than the "normal" focal length for a given format, producing a narrower angle of view and greater magnification.
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