Saturday, July 1, 2006
When the lens weighs more than the camera body, it's best to mount the lens rather than the camera on the tripod for balance and to save stress on the lens mount. Many telephoto lenses, zoom or fixed-focal length, come with tripod mounts. The best mounts allow you to switch between horizontal- and vertical-format shooting without adjusting the tripod; just loosen the knob and rotate the camera/lens combo in the mounting ring. The mounting ring makes a great left-hand grip for handheld shooting, too.
One annoyance with heavy zoom lenses is that they tend to extend as you carry them, thanks to gravity. Some lenses have a zoom lock, which can be used to lock the lens at its shortest length for easier oncamera carrying.
Fixed Vs. Variable Maximum Aperture
As with fixed-focal-length lenses, faster maximum apertures let you shoot in dimmer light, or at faster shutter speeds. They also provide faster autofocusing performance and easier manual focusing. If you do a lot of low-light or handheld shooting, you'll want a fast lens, but keep in mind that fast lenses cost a lot more than slower glass.
A consideration unique to zoom lenses is variable maximum apertures. Constant-aperture zoom lenses have a fixed maximum aperture: if you set, say, a 70-200mm ƒ/2.8 zoom to ƒ/2.8, it stays at ƒ/2.8 at all focal lengths. Most lower-cost zooms have variable maximum apertures: if you set a 70- 210mm ƒ/4-5.6 zoom to ƒ/4 at the 70mm setting and zoom to its longest focal length, the aperture will change to ƒ/5.6. TTL metering automatically compensates for this, but if you're using a handheld meter, you'll have to take it into consideration. Constant-aperture zooms also maintain focus when zoomed; variable-aperture zooms must be refocused when you change focal lengths. Autofocusing systems automatically handle that, but when focusing manually with a variable-aperture zoom, be sure to focus at the focal length you're going to use for the shot.
Because the image sensors in many D-SLRs are considerably smaller than a full 35mm film frame, a lens designed specifically for them doesn't have to project as large an image circle. Thus, lenses specially designed for such cameras can be smaller and more efficient on these cameras than lenses designed for 35mm SLRs. The drawback is that these "APS-C"-class lenses (so named because these image sensors are about the size of an Advanced Photo System C-format image frame) can't be used on full-frame cameras, in part because vignetting will occur. Lens manufacturers identify these lenses via code letters in the lens name: Canon designates its "APS-C" zooms "EFS"; Nikon labels its "DX"; Pentax, "DA"; Sigma, "DC"; and Tamron, "Di II." All lenses for Olympus' Four Thirds System D-SLRs are digital-only; they use a different mount than the company's old OMsystem film cameras. Sigma and Tamron also make lenses designated "DG" and "Di," respectively; these are optimized for digital SLRs, but provide a standard 35mmformat image circle and so can be used with all SLRs, film and digital, for which a mount is available.
What's Best For You?
The best telephoto zoom lens for you depends on your shooting needs. If you bought an entry-level D-SLR with an 18-55mm standard zoom, one of the 55-200mm zooms is a logical next step. If you like to photograph birds and other wildlife, or sports-action, a longer lens is a better choice: 300mm and up. If you prefer wide-angle work, you don't need a tele-zoom.
A good plan is first to decide what focallength range is most useful to you and the kind of photography you do. Then check out what's available in that focal-length range. The accompanying chart, while not complete, offers a good idea of what's out there. Consider the features we've discussed and how each might help your photography.
If you can afford it, get one of the pro zooms. They're faster, perform better and last longer. But even an entry-level, major-brand tele-zoom lets you get shots you can't get with shorter focal lengths and provides a great way to break into telephoto photography.
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