When you own a Single Lens Reflex (SLR) camera, you have the option of purchasing more than one lens for various purposes. This article explores some characteristics you should consider once you've decided on the type of zoom lens you plan to purchase, including Internal Focusing, Image Stabilization or Vibration Reduction, Constant vs Variable Aperture, Macro Focusing, Low-Dispersion Glass, Apochromatic Lenses, Aspherical Elements, Tripod Mounts for Long Lenses, Plastic vs Alloys, Size, Weight, and Price.

Internal Focusing (IF)
Characteristics: The movement of the lens for focusing purposes is internal to the lens body, so no lens ring will cause the lens to twist out and interfere with the orientation of any filters. (Some filters, such as linear polarizers or graduated neutral-density filters require stationary positioning in order to deliver predictable results.) This internal movement also allows the lens weight to stay balanced, which helps the balancing of heavier lenses on a tripod. Image quality of newer IF lenses is slightly better than non-IF lenses due to technological advances made that correct some aberrations.

Image Stabilization (IS) or Vibration Reduction (VR)
Characteristics: Most types of lenses above come in two flavours: either with image stabilization, or without. Built-in stabilization allows you to get sharp hand-held images when lighting conditions are poor, or you're shooting from a moving vehicle. On the lenses that offer this capability, there is usually a switch that lets you turn the feature off, thus saving batteries when using a tripod. Beware the cost of these lenses, though: you can expect to pay 2-4 times as much for a lens with image stabilization than the same lens without.

Constant vs Variable Aperture
Characteristics: Variable aperture accounts for the majority of zoom lenses, and causes the maximum aperture to decrease as you zoom from wider focal lengths to telephoto focal lengths. The advantage of variable aperture zoom lenses is that they're cheaper to maufacture and are typically lighter and more compact than contact aperture lenses.

Constant aperture lenses provide a static maximum aperture regardless of the focal length you're zoomed to. This better enables you to achieve narrow depth of field so as to isolate your subject from your background. These lenses typically let more light in, which lets you shoot sharp photos in dimmer lighting conditions.

Macro Focusing
Characteristics: Macro lenses allow the camera to be very close to its subject and still focus sharply. Macro lenses are ideal for close-up shots of flowers, leaves or insects, and sometimes double as portrait lenses. They typically have a reproduction ratio of 1:4 or better, which means the image sensor will capture the subject at 1/4 its original size or larger.

Low-Dispersion Glass
Characteristics: Some lenses are made with high-quality glass specially manufactured to more accurately transmit the full visible colour spectrum, called low-dispersion glass. You may also hear of ED (extra-low dispersion), SLD (super-low dispersion) or UD (ultra-low dispersion), which really just means truer colours.

Apochromatic Lenses (APO)
Characteristics: Apochromatic lenses have high colour fidelity and sharpness and should be used by anyone obsessed with getting to the most accurate colours possible.

Aspherical Elements (ASP)
Characteristics: Aspherical lenses are made with a different curvature than traditional spherical lenses to correct for distortions at the edge of the image. They are particularly useful in wide-angle lenses where vignetting or distortion can be seen in the corners of the image.

Tripod Mounts for Long Lenses
Characteristics: Lenses that are bigger and heavier than your camera body should have a tripod mount built-in. By attaching the lens to the tripod instead of the camera, you will remove tension on the camera-to-lens mount joint, and you will improve tripod balancing in the field. In fact, without a tripod mount, some larger lenses will cause the tripod to topple, thus making tripod use impossible without a lens mount. It is possible to purchase a separate adapter bracket from some manufacturers if the lens itself does not have one.

Plastic Material vs Alloys
Characteristics: Plastic lenses built recently typically use high-tech materials that are very durable and much lighter than metal alloy lenses. So although it used to be said that plastic lenses are cheap and unreliable, that is no longer true today.

Size
Characteristics: Zoom and telephoto lenses are getting smaller and lighter each year. In general, the larger the focal length of the lens, the larger the lens will be physically.

Weight
Characteristics: The heaviness of a lens may be prohibitive to you, depending on your physical fitness level and how long you'll be away from home. Plastic lenses are quite durable and are definitely lighter than equivalent metal lenses, although ultra-lightweight metal alloys are reasonable as well.

Price
Characteristics: In general, no matter which lens you're looking at, prices vary dramatically based on lens quality. Professional lenses will cost you thousands of dollars, while a compact modest starter zoom lens will be only a hundred dollars. You'll pay extra for some of the characteristics discussed in this article, such as fast constant apertures, high quality glass, image stabilization, and quick auto-focus. Remember, though, that lens price is a large investment: lenses will long outlast your camera body, and is largely responsible for the quality of your images (much more so than the camera body itself)!