Buy a camera lens

If you purchased your digital single-lens reflex (DSLR) as part of a kit, you already have a basic lens that takes pretty good pictures. However, part of the attraction of this type of camera is that you can switch out lenses to get the best shot in any situation. From powerful zooms that get you up close to high-speed lenses that specialize in low-light settings, you have plenty of options for your second lens. While some lenses may go for more than you spent on your camera, you don't have to pay a lot to get a great lens. The real question is: how do you find the right one for your needs?"

Understanding glass

To understand what makes one lens different from another, you first need to be familiar with a few basic concepts.

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If you purchased your digital single-lens reflex (DSLR) as part of a kit, you already have a basic lens that takes pretty good pictures. However, part of the attraction of this type of camera is that you can switch out lenses to get the best shot in any situation. From powerful zooms that get you up close to high-speed lenses that specialize in low-light settings, you have plenty of options for your second lens. While some lenses may go for more than you spent on your camera, you don't have to pay a lot to get a great lens. The real question is: how do you find the right one for your needs?"

Understanding glass

To understand what makes one lens different from another, you first need to be familiar with a few basic concepts.

Focal Length Technically speaking, a lens's focal length, represented in millimeters, is the distance between the rear element of the lens and the focal plane, where the parallel beams of light entering a lens converge to a point. This matters to you because the focal length determines the lens's viewing angle. The shorter the focal length, the wider the field of view, and the more of a scene your camera can capture. As you increase the focal length, the field of view narrows-so you see less of the scene-and objects appear magnified in relation to their environment.

In the world of traditional 35mm film, focal lengths ranging from 16mm to 35mm are generally considered wide-angle. A 50mm lens is referred to as "normal"-because it comes closest to covering the same field of view as the human eye-and anything over 100mm is considered telephoto. However, focal length is a bit more complicated when it comes to DSLRs. That's because the cameras' image sensors are smaller than 35mm film, so they crop out some of the image and give the effect of a longer focal length. To get a sense of how this discrepancy will impact the viewing angle of your lens, you have to multiply the digital camera's crop factor-which you'll find in its manual-by the focal length of the lens. A Nikon D40, for example, has a crop factor of 1.5X. As a result, a 35mm lens has a field of view equivalent to that of a 52mm lens when placed on the D40.

Aperture The other key component in evaluating a lens is its aperture. The aperture is one of the mechanisms that controls the amount of light that passes through the lens to the image sensor. The aperture is usually referred to in terms of f-stops, and is represented by a number such as f/2.8. The smaller the number, the larger the opening, and the more light it allows into the camera. Because it collects more light, a lens that opens to a wide aperture lets you maintain faster shutter speeds in low light-this can be essential for obtaining sharp images from a handheld camera. Wider apertures also provide more creative control by giving you the option to throw backgrounds out of focus.

The speed of a lens is described by its maximum aperture. Some lenses, for example, max out at f/4.5, while others can open all the way to f/2 or wider. If you're looking at a zoom lens, which has a range of focal lengths, you'll see the maximum aperture listed as a range, such as 55-200mm f/4-5.6. This lens zooms from 55mm, with a maximum aperture of f/4, to 200mm, with a maximum aperture of f/5.6. This means your camera will choose a slower shutter speed as you zoom in.

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