The most important thing to look for in a lens is the quality of the lens and the manufacture. You want to buy a lens that is well-made, using high-quality parts. Although it can be foolhardy to depend solely on brand names to choose a product, a few names in the camera business stand out for their production of lenses. These include Canon, Nikon (Nikkor) and Olympus. Although you could be very happy with the quality of a lens from a manufacturer different from these we’ve just mentioned, nearly any lens made by those companies is going to be very well-made.
You are also going to have to know whether the camera that will be used with the lens for which you are shopping is a DSLR or has an interchangeable lens body like an MILC (Mirrorless Interchangeable Lens Camera). Each type of body uses a different mounting system, and a lens that fits one type of digital camera will not fit on the other without an adapter. Also, a lens made for one brand of MILC or DSLR will not fit on a body made by a different manufacturer. For instance, a Canon lens will not fit on an Olympus camera body.
It’s also important to know what lenses the photographer already has in their collection, and which they do not. Lenses with short focal lengths (35mm, for instance) are great for closeups and getting up close to the subject. Lenses with long focal lengths (like around 105mm) are usually used for taking photos of objects much further away. Lenses provide almost a telescopic view of the scene at longer focal lengths. The difference in focal length also determines the field of view. You will see much greater of a distance from side to side in a photo taken with a 35mm lens than you would in a shot done with a 105mm model. One of the beauties of any camera that uses interchangeable lenses is that no one lens is going to be perfect for every situation. Having more lenses in your repertoire is only going to give you that many more opportunities to get the perfect shot.
The other really important thing to be aware of when purchasing an interchangeable lens is the aperture, or “f/stop” of the lens. This is a measure of how much light is allowed through the lens at one time. Aperture is usually expressed as an f/stop rating between f/1.8 and f/45. The lower the number, the more light that passes through and the exposure time will be decreased. This is what you would be looking for in photographs of sporting action or anywhere else where quick movement might cause blurriness. Such lenses are also better for low-light situations where every single bit of light you can grab is needed to make the photo come out its best. The problem with fast lenses, however, is their additional cost and weight.
Get a variety of lenses with different focal lengths and make sure they are manufactured by a first-rate manufacturer. Finally, get a lens that has an aperture comparable with the ones already in your photographic arsenal.
A selection of different lenses is like a toolbox to photographers. Build up your collection today.