Does Lens Speed Matter?

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.

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.

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!

Today, 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.

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.

Using 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.

There’s Something To Be Said For Slower Lenses
Slower lenses are smaller, lighter and much less costly than faster lenses of equal focal length, making it possible for many more photographers to access longer focal lengths. Slower lenses also generally take smaller-diameter filters, which saves you money if you use a lot of filters.

With a slower lens, however, the viewfinder image will be dimmer, making composition and manual focusing more difficult. Autofocusing will be slower (and with some SLRs, not possible), and you’ll have to use a slower shutter speed or higher ISO in any given light level.

Bottom Line
When budget and logistics permit, faster lenses can give you better photos because they let you use lower ISOs for better image quality, provide faster autofocusing performance and let you use faster shutter speeds in a given light level. When fast lenses are too costly or bulky, slower lenses allow you to obtain focal lengths you couldn’t have otherwise. But for those whose living or reputation depends on image quality, faster lenses are well worth the cost.

Click here for A Fast Lens Sampler PDF.