Camera Lens

A good lens is the most essential part of your photographic equipment package. When picking out a camera lens, there are a lot of factors to consider:

Aperture
Generally, when buying lens, one specification that will be mentioned is the maximum aperture. Generally, the nicer a lens is, the bigger the maximum aperture will be. Remember, the smaller the number, the bigger maximum aperture is. When dealing with zoom lenses, they generally provide two numbers for maximum aperture. Really, these two numbers represent the range of maximum apertures of which the lens is capable. The reason being that with a zoom lens, the maximum aperture changes as you change the focal length. For example, with a 80mm to 200mm zoom, you will be able to achieve a bigger aperture opening at 80mm than you will at 200mm.

Focal Length
Though technically focal length is focusing distance, when dealing with camera lenses, what this figure designates is the angle of view or magnification of the lens. The bigger the focal length, the more magnified the image. Conversely, the smaller the focal length, the wider area your camera lens will capture.
 

Zoom vs. Fixed
Another factor to conisder is whether to get a fixed focal length lens, or a zoom lens. Fixed focal length lenses tend to produce better results, while zoom lenses offer more flexibility. I would recommend getting a fixe dlens simply because good fixed lenses are much more affordable than good zoom lenses. (Getting a zoom lens that produces comparable results requires buying more "professional" grade equipment).

Filter Size
Also, when buying a lens, the manufacturer will specify the filter size. This figure doesn't really affect the performance of the lens. However, it should be considered when making a purchasing decision for a few reasons. First of all, if every lens you have requires a different filter size, you will either need a large quantity of duplicate filters, or a filter system that utilizes a filter holder and interchangeable filters. Secondly, it is also important to note that the larger the filter size, the more expensive the filter will be. For example, a 72mm filter will often cost twice as much as a 58mm filter.

Lens Size
Length and weight are other factors to consider simply because the longer and heavier the lens is, the bigger and heavier your camera will be. Often times, when using high quality telephoto lenses, the lens will easily outweigh a 35mm camera.

Most importantly, when buying a lens make sure it is from a reputable company. If you have doubts, check to see if the lens in question has received good reviews. There are a number of web sites and magazines that review photography equipment, and finding reviews shouldn't be difficult.