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Tuesday, February 25, 2014

Super Telephoto Zooms

We decipher the technobabble behind these nature photography mainstays

Labels: Gear
The advantages of APS-C lenses are that they can be designed to be smaller and perform better with smaller sensors. But if you intend to go full-frame some day, you'll be better off buying full-frame lenses now, even if you currently use an APS-C camera—you won't be able to use APS-C lenses on your new full-frame camera and take full advantage of its sensor's megapixels due to the crop factor.

Focal-Length "Breathing"
With many zoom lenses (especially those using internal focusing), the maximum focal length decreases as you focus closer—in some cases, a 70-200mm zoom winds up with a maximum focal length of maybe 140mm when set at 200mm and its closest focusing distance. For most purposes, this isn't a big deal. If you're at the lens' minimum focusing distance, a really long focal length isn't as important as with distant objects. If you do insect and flower photography, consider the minimum focusing distance and magnification: If your 70-200mm lens focuses down to 0.25X, it doesn't really matter if it's doing that at 200mm or 140mm; you're still getting 0.25X. (Of course, this focus breathing means you have less working distance—less space between you and the insect—but at these distances, that's not as critical as at true macro shooting distances.

Internal focusing offers its advantages. First, the lens doesn't change physical length during focusing, good for balance. Second, the front element doesn't rotate during focusing, so polarizers and graduated and other orientation-sensitive filters maintain their orientation. Note that while internal-focusing lenses don't rotate or extend as they focus, many do rotate and extend physically as they're zoomed.

Zoom Method
Most zooms today use a zoom ring, which you rotate to change focal lengths. But with some, you push or pull the zoom control rather than rotate it. The push-pull type is probably more prone to sucking dust into the lens, but we haven't found that to be a big problem. So, mostly, it's a matter of personal preference. Some photographers feel more comfortable with the rotating ring, others, with the push-pull control.

Exotic Elements
Most telephoto zoom lenses incorporate low-dispersion elements to minimize the effects of chromatic aberrations, improving image quality. They have designations such as LD, SLD, ED, ELD, HID, ULD and the like, depending on manufacturer and degree of correction. Fluorite elements are even more effective at compensating for chromatic aberrations, but also very costly, and are found only in some high-priced lenses. Lenses with low-dispersion elements are "better" than those without, but you should always test a telezoom before buying (you can rent many for a couple of days), rather than just depend on the presence and number of such elements.

Aspherical elements correct spherical aberration, which is more of a problem with wide-angle lenses, so they aren't often found in telezooms.


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