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Tuesday, May 1, 2007

Does Lens Speed Matter?


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

Tamron SP AF17-50mm ƒ/2.8 XR Di-II LD Aspherical (IF)
Better Is Bigger, Right?

As indicated, the primary drawbacks to fast lenses are bulk and cost. Why are fast lenses so much bulkier than slower ones? Because a larger maximum aperture requires a larger-diameter lens, and because additional and more costly elements are needed to correct the aberrations and distortions inherent in larger-diameter optics. For example, the aforementioned 400mm ƒ/5.6 lens contains seven elements in six groups, while the 400mm ƒ/2.8 contains 17 elements in 13 groups.

One thing that helps to keep lens size down is internal focusing, where smaller internal elements move during focusing instead of those big front ones. Internal focusing also speeds up focusing, provides better balance, allows for shorter minimum focusing distances and prevents the front element from rotating during focusing—a big plus when using orientation-sensitive lens attachments like polarizers and graduated filters. Many fast lenses feature internal (or rear) focusing.

Why do fast lenses cost more? Because those larger elements are much more costly to produce and correct, and because far fewer superfast lenses are sold. Superfast lenses tend to employ the best technologies and materials. So while you’re paying more, you’re getting even more for your money than just the many fast-lens shooting advantages.

Shorter Focal Lengths
Not all fast lenses are superlong monsters. In fact, the first fast lenses were "normal" ones. The 50mm ƒ/1.4 was the "class" choice of many a photographer when I started in photography, much favored over the slower 50mm ƒ/2.0 (even though ƒ/1.4s cost a lot more and weren’t as sharp wide open as ƒ/2.0s). Years later, Canon offered the now-discontinued EF 50mm ƒ/1.0 for EOS models, and Leica still produces the pioneering 50mm ƒ/1.0 Noctilux-M for its 35mm rangefinder cameras. An ƒ/1.0 lens transmits twice as much light as an ƒ/1.4. That’s fast!

Olympus Zuiko Digital ED 300mm ƒ/2.8Today, less than $500 will buy you major-brand ƒ/2.8 wide-angle and wide-to-short tele-zooms, 85mm ƒ/1.8 portrait telephotos and 50mm ƒ/1.4 fast "normal" lenses. The popular 70-200mm ƒ/2.8 action zooms start just over twice that. These all provide a big performance advantage over their slower counterparts, without the multi-thousand-dollar price tags of the fastest glass.

It’s All Relative
"Fast" is relative. While ƒ/2.8 sends the same amount of light to the film or image sensor regardless of lens focal length, a maximum aperture of ƒ/2.8 is considered superfast for a supertelephoto, but very slow for a 50mm "normal" lens. Today, you can get wide-angles for film and D-SLRs as fast as ƒ/1.4, "normal" and short-telephoto lenses as fast as ƒ/1.2, and zoom and 1:1 macro lenses as fast as ƒ/2.8, along with the aforementioned ƒ/2.8 and ƒ/4 supertelephotos.

You also have to consider image-sensor size with D-SLRs. A given focal length produces a different field of view with different-sized image sensors because smaller sensors "see" less of the image produced by the lens. Thus, a 100mm lens used on a D-SLR with an APS-C image sensor produces a cropping equal to that of a 150mm lens on a 35mm camera (1.5x crop factor), and used on a Four Thirds System D-SLR, produces a cropping equal to that of a 200mm lens on a 35mm SLR (2x crop factor).

Apertures don’t change when you mount the lens on different-format D-SLRs, just the cropping. Thus, the Olympus 300mm ƒ/2.8 lens for Four Thirds System cameras is effectively equivalent to a 600mm ƒ/2.8 on a 35mm SLR or full-frame-sensor D-SLR—a full stop faster than 600mm ƒ/4 lenses. Of course, those 600mm ƒ/4s effectively become 900mm ƒ/4s when used on APS-C sensor D-SLRs, while 400mm ƒ/2.8s become 600mm ƒ/2.8s—the same as the Olympus 300mm ƒ/2.8.


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