What you need to know to pick the perfect lenses for your digital camera
Fisheye lenses are extreme wide-angles (often with focal measurements in the teens) that deliver an angle of view greater than 100° and often as much as 180º. Not all superwides are fisheyes, but all fisheyes are superwide. Some fisheye lenses actually create a circular image in the center of the frame, making it possible to create circular, full-sky images.
Lenses control how much light passes through by way of an aperture. Every lens has a range of these ƒ-stops, from maximum (as wide open as the lens can be, corresponding to a small number like ƒ/2) to minimum (as stopped down as a lens can close, with a large number like ƒ/32).
The maximum aperture, known as the “speed” of a lens, is critical. Working in low-light situations in which a fast shutter speed is needed (stopping sports action, for instance), a larger maximum aperture is worth its weight in gold. Fast lenses—say, ƒ/2 or even ƒ/1.4—are more expensive than slower lenses because they’re more versatile. They’re also often larger because they require bigger elements. Consequently, many lens makers incorporate variable maximum apertures in their zoom lenses, from ƒ/3.5 at the wide end to ƒ/5.6 at the telephoto end, for instance. A variable-maximum-aperture zoom is usually lighter and less expensive than a fixed-aperture model.
DECIPHERING THE CODES
When shopping for lenses, you’re bound to notice a variety of acronyms and cryptic codes appended to lens names. Codes like SD, ED and T* refer to optical characteristics and coatings that are designed to deliver better color, contrast and sharpness. Coatings on internal elements minimize lens flare, while front-element coatings make glass scratch- and dust-resistant.
If you see codes like IS, OS and VR, you’re looking at a lens with image stabilization. The manufacturers’ terms are different, but the functions are the same—to reduce the effects of camera shake for sharper images when handholding, sometimes by as much as two full stops of steadying power (so instead of seeing camera shake at 1/60 sec., you can hold steady even at 1/15 sec.). Everything comes with a price, though, and stabilization is often paid for in the heft and cost of a lens.
HSM and USM refer to autofocus motors. The hypersonic and ultrasonic designations are designed to be, clearly, really fast. If focusing speed is critical, say, for sports or fast-moving wildlife, look to these lenses.
The term ASPH is short for aspherical. Instead of a wide-angle image focused correctly in the center and distorted at the edges, aspherical elements help keep sharpness constant. Wide-angle lenses tend to distort straight lines into curves, and for that, rectilinear designs intervene to keep straight lines straight, as well.
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