Does Lens Speed Matter?

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

By Mike Stensvold

When shopping for a new lens, you might encounter the desired focal length (or focal-length range, in a zoom lens) in more than one speed. For example, one camera manufacturer’s lineup includes 400mm ƒ/2.8, 400mm ƒ/4 and 400mm ƒ/5.6 supertelephotos. The ƒ/2.8 is 4.5 times larger in volume, 4.2 times heavier and costs $5,000 more than the ƒ/5.6. Is it worth it? Many wildlife and action photographers think it is.

Faster lenses let in more light, bestowing a wide range of benefits:

1 You can use a faster shutter speed in any given light level, handy with moving subjects or when handholding the lens.

2 You can shoot with a lower ISO in a given light level to minimize grain in film images and noise in digital images.

3 You get a brighter image in the viewfinder for easier composing and manual focusing.

4 You get faster autofocusing—that’s a big reason why action pros use faster lenses.

Canon EF 400mm ƒ/4 DO IS USM 5 You get autofocusing when using a 2x teleconverter (the two-stop light loss with the 2x converter renders AF systems unusable with slower lenses on many cameras).

6 Since most lenses are sharpest a stop or two down from their maximum aperture, you can use a faster lens at its sharpest aperture and get the same aperture as when using a slower lens wide open.

7 Besides letting in more light, a wider maximum aperture allows you to limit depth of field severely, handy when you want to isolate a subject from a busy background.

There was a time when fast lenses were noticeably less sharp than slower ones when both were used wide open. But modern design technology and materials have changed that, and today’s major-brand fast lenses provide excellent performance, even wide open. In fact, most of today’s fast lenses were designed specifically for professional photographers, and wildlife and action pros love them.