Buyer's Guide 2007: Lens Strategy
How to select the right mix of lenses for your photography
DESIGNED FOR DIGITAL
Before digital SLRs reached ubiquity, lenses for 35mm cameras provided enough coverage for a 35mm film frame-more than enough when used on a D-SLR with a smaller imaging sensor. Manufacturers realized that this overkill meant photographers with digital SLRs were using larger and heavier lenses than they actually needed, so they began to make lenses designed for smaller digital sensors. Now those digital lenses are common.
Digital lenses often offer a faster lens in a smaller package, but they also eliminate the telephoto factor that comes from using a film lens on a camera with a smaller digital sensor.
Digital lenses are also designed to perform better with a digital sensor. Image sensors require that light strike them at a near 90-degree angle, whereas photographic film can handle light coming at more obtuse angles. Digital lenses are designed to collimate the light, ultimately improving sharpness, contrast and color rendition, and minimizing ghosting. But beware; a designed-for-digital lens won't work on a 35mm film camera-even if they share the same mount. Because digital lenses cover a smaller area, the edges of a film frame would vignette if a digital lens were used on a full-frame digital or film SLR.
How is one lens faster than another? The term "speed" is a bit deceiving because it doesn't have anything to do with how fast a lens focuses or zooms. The speed of a lens refers to its maximum aperture. Apertures, also called ƒ-stops, control the amount of light let into the camera. These numbers are uniform across all manufacturers and focal lengths. A faster lens has a greater maximum aperture, which is denoted by a smaller
ƒ-number-like ƒ/1.4 or ƒ/2. This might not matter to all photographers, unless they need to shoot in situations where a very fast shutter speed is required (thus needing a larger maximum aperture), in low-light conditions or when a minimal depth of field is desired.
Some zoom lenses have a variable maximum aperture, which is denoted by a range like ƒ/4.5-5.6. This means that at the lens' shortest focal length, the maximum aperture will be ƒ/4.5, while at the telephoto end, the maximum aperture is ƒ/5.6. These variable maximum aperture lenses sacrifice speed at the telephoto end, but they also make for smaller and lighter zooms and less sticker shock at purchasing time.
The hardest part about lens shopping can be deciphering the abbreviations. What on earth could LD, DX, APO and IS have to do with good glass? What's more, why do different manufacturers each use different terminology?
The terms IS and VR are used to denote Image Stabilizer and Vibration Reduction lenses respectively. Canon calls it IS while Nikon calls it VR, but both technologies achieve the same result-reducing camera shake to make sharper pictures when handholding the camera and using slower shutter speeds. It's not a replacement for a tripod, but it can help out in a pinch or when using a long lens that amplifies every tiny tremble.
Abbreviations such as ED, LD, SLD, APO and T* identify low- or ultra-low-dispersion glass coatings that help to improve the color, contrast and sharpness that a lens will provide. The terms "aspherical" and "rectilinear" are commonly found on wide-angle lenses. Aspherical glass is designed to maintain critical sharpness and color rendition throughout the coverage area, and rectilinear lenses are designed to prevent the barrel distortion commonly seen at the periphery of a wide-angle frame.