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March 22, 2007
Tweaking the Zoom
By IAN AUSTEN
MANY digital photographers are taking the plunge and upgrading to a single-lens reflex camera, in part because the lenses on such cameras are interchangeable. But one trip to a well-stocked camera store might convince some novices that this feature is not so attractive after all.
Nikon alone sells 51 different lenses in the United States for its cameras, while Canon has 54 offerings. Still not enough? Several companies, notably Tamron and Sigma, have their own complete lineups of lenses to fit most brand-name cameras.
But the bewildering array of lenses is not a marketing trick to promote sales through confusion. It reflects the fact that it is impossible for lens makers to produce a single lens, or even a small number of lenses, to meet every photographic need.
“There are always compromises,” said Steve Heiner, senior technical manager for Nikon USA. “We’re having to deal with basic physics in a lot of ways that put limits on what we can do.”
Zoom Improvements
One factor has improved the situation for shoppers compared with the predigital days: robots. The image a lens creates is as much a product of its assembly as its design. Robots now make it possible to build inexpensive lenses with a precision that could once be achieved only through minute, and costly, manual adjustments.
Better optical design software and the substitution of plastics for brass and aluminum have also helped lenses get cheaper and better. Zoom lenses are no longer poor cousins, optically speaking, to the conventional lenses they replace. “The development of the zoom lens has been one of the most impressive things I’ve seen over the last 25 years,” Mr. Heiner said.
Zooms, of course, allow users to change the focal length — the number that is always expressed in millimeters in a lens’s name — with the twist of a ring. That allows one lens to do the work of many. Most digital cameras come with a zoom lens that is reasonably wide angle at one extreme and moderately telephoto at the other, although the telephoto setting is typically not sufficient for sports or nature photography.
Film Lenses vs. Digital
In the age of the 35-millimeter film camera, the field of view captured by a lens with a focal length of about 50 millimeters was deemed to be roughly the equivalent of that captured by the human eye. Anything above that was a telephoto lens; anything substantially lower was wide angle.
But most digital S.L.R.’s have imaging sensors that are smaller than 35-millimeter film, so a lens with a smaller focal length can capture the same subject area. As a result, the new normal, at least for focal lengths, is about 35 millimeters.
While advertisements often say a digital camera lens’s focal length is “35-millimeter film equivalent,” that bit of shorthand is slightly misleading. Certainly the subject area taken in by, say, a 50-millimeter lens when it is used on a Nikon digital S.L.R. is the same as that of a 75-millimeter lens on a Nikon film camera. But the actual focal length affects other properties of the lenses — which can be both good and bad.
Lenses with longer focal lengths generally have a shallower in-focus area than shorter focal-length lenses, which can make it hard to shoot a fast-moving subject. A photographer using a 200-millimeter lens on a digital camera gets the narrow field of view offered by a 300-millimeter telephoto on a film camera, while avoiding the shallower focus of the bigger lens — as well as its higher cost, larger size and greater weight.
But shallow focus, or limited depth of field, can also be a tool for blurring out unfortunate backgrounds and creating a sense of depth. A large maximum opening for the lens (which, counterintuitively, is represented by a small f-stop number, like f1.4) can reduce depth of field. The one big compromise shared by most zooms is that they usually offer relatively modest maximum lens openings of f4 or smaller.
Super Zooms
Most lens makers have taken advantage of the smaller digital camera sensors, creating lenses that, in general, are smaller and lighter while sometimes offering zoom ranges that exceed anything ever offered for film. Sigma and Nikon have “super zoom” lenses that go from a wide-angle setting of 18 millimeters to a robustly telephoto 200 millimeters, while Tamron offers an 18-250 millimeter model.
All three of these lenses can more than cover the focal length needs of most photographers and are remarkably compact. So has the perfect lens finally arrived? Alas, probably not. While Mr. Heiner said he brought an 18-200 millimeter along when he was traveling, he acknowledged that it was optically inferior to other zooms that did not attempt to span such a wide range.
Buying a second lens to supplement, rather than replace, the zoom that came in the camera box can result in better quality, and can also be cheaper. This month, Nikon introduced a 55-200 millimeter lens that will sell for $270 but offers features similar to the $750 18-200 super zoom.
Another problem with super zooms is their modest light-gathering ability. The Tamron super zoom, for example, has a maximum opening of just f6.3 at its longest focal length setting. Parents hoping to use it to capture the action at an evening soccer match will most likely be disappointed.
Canon and Nikon offer several lenses with systems that enlist electronics and moving optics to offset blurring from shaky hands at slow shutter speeds, including Nikon’s 18-200 millimeter and its new 55-200 millimeter. (Sony builds its antiblur system into the camera, rather than the lens.)
The Independents
It is an open secret that companies like Sigma produce lenses under contract to brand-name camera companies. But their own lenses were once viewed as second rate. Most lens tests today show that their products perform about as well as, and in some cases better than, comparable and often more expensive brand-name offerings.
The one potential drawback to the outsider lenses is created by the camera makers. Camera companies will not reveal anything about the electronics that link lenses with their cameras’ autofocus and exposure systems, forcing the independents to reverse-engineer the answers.
As a result, slight model changes in cameras have left independent lenses unusable, or the lenses have caused problems with the cameras’ operation. Both Sigma and Tamron have recalled and altered lenses in such cases at no cost to owners. They have also designed lenses to allow for updates to be made with software.
Time for Primes
Although the zoom lens now reigns, nearly every lens maker still offers fixed focal length, or prime, lenses.
“Prime lenses provide the highest optical quality you can get,” said Chuck Westfall, a technology spokesman for Canon in the United States. “They are the benchmark.”
While optical differences between primes and zooms can sometimes be lost on nonprofessionals, primes do have other benefits. Many have large maximum openings that allow indoor photography without flash, and they can be more compact than a zoom.
In some cases, being the ultimate in optics does not come with a correspondingly high price. Mr. Heiner said he often recommended that amateur users supplemented the zoom that came with their camera with a 50-millimeter f1.8 lens, the kind of prime lens that was once shipped with most film cameras. Not only is it one of Nikon’s best optical designs, he said, but “at about $100, it’s also our best value.”
