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Tuesday, May 1, 2007

Does Lens Speed Matter?

Pros love their fast glass. Maybe they're onto something.

200-400mm ƒ/4G ED-IF AF-S VR Zoom-Nikkor

Fast Zooms

Zoom lenses come in two varieties: constant aperture and variable aperture. If lens speed is important to you, you want a zoom that’s fast throughout its focal-length range (i.e., constant aperture). Variable-aperture zooms cost less, but don’t give you the benefits of speed at their longest focal lengths. For example, a 17-35mm ƒ/2.8 constant-aperture zoom costs around $1,500, while a 17-35mm ƒ/2.8-4.0 costs around $300. But at the 35mm setting, the variable-aperture lens is a stop slower than the constant-aperture zoom; it transmits just half as much light at 35mm. You have to decide whether the extra speed at the long end of the range is worth the extra cost for your photography.

While zoom lenses are versatile in providing a range of focal lengths in a single unit, even the fastest ones aren’t as fast as the fastest single-focal-length lenses. For example, the fastest zoom lens that includes 24mm is an ƒ/2.8, while 24mm fixed-focal-length lenses are available down to ƒ/1.4. The fastest zoom lens containing a 500mm focal length is an ƒ/6.3 (at the 500mm setting), while fixed-focal-length 500mm lenses are available as fast as ƒ/4.

Sigma APO 50-150mm ƒ/2.8 EX DC HSMUsing Fast Lenses
Shorter-focal-length fast lenses are small enough to be used handheld, just as you’d use their slower counterparts. Bulky, superfast telephotos are best used on tripods. They’re too unwieldy to hand-hold, and even if you could hold one steady, holding it for more than a few moments would prove fatiguing.

A number of big-lens users I know work with monopods. Doing so steadies the lens and relieves the arms of the need to support the lens for extended periods. A tripod is steadier, but a monopod is easier to haul around, and it works. Superfast supertelephoto lenses come with their own tripod mounts; because they’re heavier than the camera bodies, the lens rather than the body is attached to the tripod to prevent undue stress to the lens mount.

If you intend to shoot action with a long lens, you might want to consider a gimbal tripod head such as those from Jobu, Kirk Enterprises or Wimberley. These provide support and balance for the lens, while allowing you to pan and tilt to track a moving subject. They’re popular with bird photographers.

Some superfast supertelephotos have built-in stabilizers. You might wonder why, since they won’t be handheld very often. Well, a stabilizer is also helpful when using a monopod or even a tripod. It allows you to use a lighter tripod, which you’re more likely to cart with you into the wilderness (that bulky tripod back in your car trunk isn’t much help). Even though the manual for my particular lens’ stabilizer instructs switching it off when using a tripod, I’ve found running the stabilizer gives me sharper images when using the lens on my lightweight tripod.

Because depth of field is very narrow at superfast apertures, it’s important to focus carefully, especially when using a long-focal-length fast lens. I focus manually for stationary subjects and use continuous AF for fast-moving ones.

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