If you're the proud owner of an amazing point and shoot style camera, this article might only be of informational value to you. But if you've got either a film or digital SLR camera you've already been confronted with the sometimes dizzying array of lens choices. Do you want ultra-wide angle, super fast action, or detailed macro prime glass? What does all of that even mean?
Well lens choices all come down to what you really want to take pictures of, because each lens has various features that are better for one type of photo or another. So what are the biggest features and things to look out for? Well, to answer that I'll try to organize everything by not feature, but by type of photography.
A good walk-around lens is something you will use the most, and want to have with you in almost all scenarios. It's the lens that you will want to take with you if you aren't really sure what you'll be shooting, and it should always have a nice wide end and a decent telephoto end. Some like 18mm-55mm, 24mm-70mm, even 28mm-105mm (If you noticed that these designations are very canon-esque, it's because I shoot with a Canon. But you can get ranges close to them no matter what brand you own).
Another thing to look for in an all-purpose lens is the maximum aperture of the lens, or the f-stop. For a good walk-around lens you can get away with maximum aperture of f/4, or even f/2.8 if you want something a little faster in low-light situations. But really, you probably want need something much faster than that for a general, walk-around type of lens.
Action and Sports
This type of photography is a combination of both great telephoto range, as well as a camera body with decent frames/second. You want to be able to capture the action and freeze it in the frame, but you also want to be able to reach up real close to the action and see it in life-size. So you'll be looking for the telephoto ranges like 100mm-300mm, or even the prime lens lengths like 300mm or 500mm. This will get you in real close on the action, and depending on the frames/second your body can shoot, will keep the action frozen in mid-stride.
Another big thing the sports photographers like is a low f-stop. With the longer lengths you're typically going to lose some stops, so you want to be able to have the lowest f-stop throughout the range of the lens. The higher priced lenses will allow you to shoot at the same f-stop throughout the range of the lens.
Macro lenses are all about close focusing capability, and large depth of field. Since you're working in such small, exaggerated proportions you need to be able to focus on things that are really close to the front of the lens. Most lenses have minimum focusing distances, where you can't focus on something if it's closer than a foot or so. But the actual specialized "macro" lenses will allow much closer focal distances, as well as large f-stops which will maximize how much of the scene you can keep in focus, even with the exaggerated distances of up-close photography.
For lenses that are specifically designed for macro photography you always want to look for the designation "Macro" somewhere on the lens body itself. There are even special prime lenses designed primarily for macro photography, in various focal lengths like 60mm, 90mm, and 120mm. These lenses give superior sharpness due to their fixed or "prime" focal lengths.
A good portrait lens is typically defined by it's ability to produce a nice soft "bokeh", or a blurry background. This is achieved by the low f-stops of the lens, like f/1.2 or f/1.8. Though too small for many of the various uses, it can excel in the portrait realm because of the soft blurriness of the background that it can produce. Many of the most popular portrait lenses are also prime lenses, and especially popular is the 50mm and 85mm ranges. These produce very life-like images that approximate the focal length of the eye.
Though I haven't covered every type of photography out there, these are some of the features and things to be looking for when you want to shop around for a new lens. First and foremost, after setting yourself a budget, ask yourself what kind of photos are you wanting to take with the lens. That will narrow down your options and ultimately help you pick the best lens for the right type of photographs you want to take!