CNET editors' digital SLR buying guide: Lens considerations

What you need to know, from the kind of user you are to current technologies.

Focal-length multiplier

We already noted that most digital SLRs use APS-size sensors, which are smaller than a 35mm-film frame. This gives rise to a confusing notion variously called focal-length magnification, focal-length multiplier, or lens conversion factor, which requires a brief lesson in camera optics:

Example: On a Nikon D200, which has an APS-size sensor and a resulting focal-length multiplier of 1.5X, a 50mm lens captures the same angle of view that a 75mm lens would on 35mm film (50mm x 1.5 = 75mm). Since we're used to thinking about our lenses in 35mm-film terms, it's convenient to say that a 50mm lens "acts like" a 75mm lens when it's used on a D200. And a 28mm acts like a 42mm, a 200mm like a 300mm, and so on. It's an oversimplification--only the angle of view changes--but we won't squeal to the optics police. However, the focal-length multiplier doesn't affect the lens distortion inherent in a wide-angle lens. In other words, on that D200, a 33mm lens would have the same angle of view as a 50mm lens, but it would still have the barrel distortion characteristics of a 33mm lens--not the normal, portrait-friendly characteristics of a 50mm model. 
Pros: Sports and wildlife shooters love the way all their lenses seem to be longer than they were on 35mm cameras. Cheaper medium-range zooms work as if they were megabucks supertelephotos. 
Cons: Landscape and architectural shooters can't get lenses that are wide enough for their subjects--or they're forced to buy a very pricey superwide lens, such as a 14mm.

If you find all this is maddeningly confusing, one option is to pony up the big bucks for a camera with a full-frame sensor, such as the Canon EOS 5D, so all of your lenses will perform just as they would on a film SLR.

"Made for digital" lenses

Another brief lesson in camera optics: A lens projects a circular image towards the sensor. This image circle must be big enough to cover the whole area of the sensor; otherwise, you'll see vignetting, which is dark corners and edges in your pictures.

Lenses for APS digital
Some manufacturers make "digital only" lenses, which project smaller image circles--just big enough to cover the APS sensor frame but not big enough for the 35mm film frame for which previous lenses were designed. In theory, the companies can reduce size and weight and save money by doing this, but these lenses won't work on your backup film-camera body or on a future digital SLR with a 35mm-size sensor. 
Manufacturers: Canon, Nikon, Pentax, Sigma

The Four Thirds design: Unlike film, digital sensors can produce unwanted artifacts when light rays from the lens strike the sensor at oblique angles. One of the main selling points of the Four Thirds format is that its lenses will refract light rays to strike the imager close enough to perpendicular to avoid problems. We think the jury is still out on whether this issue is really significant or not. 
Manufacturers: Olympus, Panasonic

Other notable lens features 

FEATURE	ALSO KNOWN AS	WHAT IT IS
Image stabilization	IS (Canon) VR (Nikon) OS (Sigma)	A mechanism in the lens detects and counteracts camera shake, reducing blur in handheld shots at slower shutter speeds. Note that Sony, Pentax and Olympus offer an alternative to lens-based stabilization by incorporating image stabilization into the camera body. This allows you to use optical image stabilization with any lens.
Ultrasonic focusing	USM (Canon) AF-S (Nikon) HSM (Sigma)	Virtually silent piezoelectric motors provide the best focusing speed and responsiveness.
Apochromatic correction	UD (Canon) ED (Nikon and Olympus) APO (Sigma) LD (Tamron)	Special--and expensive--glass is used in one or several of the lens elements to counteract an optical defect called chromatic aberration, which can noticeably reduce sharpness, especially in telephoto lenses, and cause color fringing along high-contrast edges.

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