Specially designed optics optimize your camera's performance
By Zachary Singer
Digital photography hasn't just changed our cameras; it's changing our lenses, too. D-SLRs and advanced compacts alike now sport specialized optics tuned for the new requirements of digital image sensors. Before we get into all that new glass, though, you'll need to know what these lenses are designed to do.
D-SLRs Physically smaller than the film they replace, the imagers in D-SLRs capture a narrower slice of your lenses' fields of view. This effect, known as magnification factor, makes a digital image taken with any given lens appear as though a longer focal length was used when compared to 35mm (it's exactly the same issue you run into when putting a roll-film back on a 4x5 view camera). Most current magnification factors run from about 1.3x to 1.7x.
With a 1.5x factor, your 28-80mm lens has an angle of view like that of a 42-120mm on 35mm. On the one hand, that's a benefit—your telephoto lenses just got "longer," but without the speed-loss penalty of a telecon-verter. An 80-200mm ƒ/2.8 acts like a 120-300mm ƒ/2.8!
Photographers who already own long lenses will see their reach increased. New buyers also can choose to get shorter lenses than they otherwise would have for the same equivalent focal length they'd use with film. (The 80-200mm lens for a 35mm film camera can be replaced with just a 50-135mm lens on a digital camera with a 1.5x factor.) That approach reduces the size and weight of the photographer's gear and opens the possibility of faster lenses as well.
Magnification factor also affects your wide-angle lenses, unfortunately, and their angles of view are narrowed, too. Your wide 24mm lens now seems more like a 36mm; to replace its angle of view, you'd need a 16mm lens on a D-SLR. Similarly, D-SLR shooters need an ungainly, ultra-wide 14mm lens to stand in for a 21mm on film. A slew of new lenses designed for digital SLRs address this problem.
Some of these lenses boast focal lengths that will seem surprisingly short and that would represent extremely wide-angle lenses on 35mm film. Such lenses would be difficult or impossible to create if they had to cover 35mm's larger area, but are achievable for the D-SLRs' small sensors. In part, this ability comes from the relative ease of designing lenses that cover smaller formats.
The new designed-for-digital optics take care of another problem as well: Unlike film, the individual photosites on imaging sensors perform better when the light comes straight in on them (think of each pixel location as a tall bucket into which you're trying to drop light, and you'll see the problem). Lenses optimized for digital collimate their light more than those designed for film, and minimize the risk of light falloff and image degradation at the corners of your photos.