What should you look for in a dslr camera lens?

The focal length is an important consideration because all DSLRs do not have the same size image sensor. For a camera with a full frame sensor, a given focal length will provide a magnification, angle of view and depth of field similar to a 35mm film SLR. For cameras with smaller sensors, the magnification is increased by a factor of from 1.3 to 1.6. The angle is more narrow and the depth is likely also affected. This is more bang for the buck if magnification is desirable, but when you need a wide angle, such as a 24 or 35 mm lens, the size factor works against you. So you will get an angle of view more like you would see with a full frame sensor at 35mm x 1.6 or 56mm (not very wide). Fortunately manufacturers have produced a new series of lenses to give an extra wide angle of view for these bodies and sensors.

Care must be taken if you plan to use the lens on more than one camera, as some DSLR lenses extend deeper into the body (closer to the path of the mirror) and may not work on all cameras. These will often have some marking to indicate that they are for digital only, which signifies that the dimensions are unique.

A good lens for any SLR camera should be sharp and have a useful range of F-stops or apertures. Sharpness is part glass quality, lens shape (Spherical vs aspherical) and number of elements and type of coatings used. Smaller f-stop numbers (or large apertures) give faster shots in lower light and help to blur the background. While the larger number (smallest aperture) determines the maximum depth under any conditions. The widest f-stop will also dictate whether your lens will still auto focus with a tele-extender. So check compatibility.

These factors all have to be balanced against the cost and proportions of the lens. A heavy lens may require that a tripod is mounted at the lens instead of the camera. They can be cumbersome and obvious, making it hard to get candid shots. Lenses for full frame bodies tend to be more in the bulky category, but can be worth it.

Some 'digital' lenses have an extra coating designed to cut ghost due to the refective nature of the image sensor. There are differing opinions as to whether of not this is necessary. But there do not seem to be disadvantages.

Auto focus speed is determined by the type of motor and gears. This also affects cost. The type of mechanism can affect how long your batteries last or whether you have enough battery power for the lens to work as intended. You need fast focus for action shots. Some lenses have image stabilization. Not all stabilization designs are equal and sometimes that feature as already in the body.

The filter size is another factor. Does the lens take a filter which is expensive and hard to locate, or is it common and maybe the same size used with your other lenses?

Check the location of the manual focus and zoom controls. Are they easy to use? Does the price include a sturdy case and a lens hood?

Kind of wordy I know, but there is much to consider.