Your camera lens has much to do with how you capture the world. So when it's time to add a new optic to your kit, how do you decide which one?
You have many things to think about: sharpness, distortion, durability, focusing speed, and cost, just to name a few. All of those factors are important. But before you get to the technical aspects, here are five considerations that might help you narrow down the list of candidates.
Stand at the corner of the busiest street in your city, and shoot ten frames with a 9mm, 24mm, 50mm, 200mm, and 500mm lens. Each group of images will tell a different story.
At 9mm, you'll capture asphalt, people, cars, buildings, and sky. Narrow the perspective a bit to 24mm, and you may have to choose between the street or the sky. At 50mm you're beginning to gather more information about a subject--maybe a particular person approaching you, or a passing car. At 200mm the details begin to dominate the story, and the background becomes less important. And when you mount a 500mm lens on your DSLR in a busy city, you probably become the focus of attention as people wonder what you're doing.
The point is, visual perspective is an important part of storytelling. When you look at your existing photographs, what's missing? Do you tend to shoot tight (close up) all the time and forget to capture an establishing midrange or long shot? Maybe your photographs are lacking the intimacy that comes from focusing on details. Look for the visual gaps in your work, and consider a lens that will help you fill them.
An all-purpose zoom lens that's typically bundled with a DSLR is good at providing a variety of perspectives from wide angle to mild telephoto. But that range usually comes at the price of a conservative maximum aperture, such as f/3.5-5.6.
What does that mean? At its wide-angle setting, say, 18mm for example, the maximum aperture is f/3.5. That's the most light the lens can pass through to your camera's image sensor. By the time you zoom to 55mm, your lens is far less bright at f/5.6. Each full f-stop (f/4, f/5.6, f/8) equals a full ISO setting (ISO 400, 800, 1600). So if you had an f/4 lens instead of f/5.6, you could lower the ISO from 1600 to 800.
When shooting in dimly lit environments with a kit zoom, you'll either have to compensate for less light coming through the lens by increasing the ISO setting (say, from ISO 400 to 800 to make your camera more sensitive to light), or by adding light using a flash, or slowing down the shutter speed (for example, from 1/60 second to 1/30 second) to let more light pass through to the sensor. If you added a brighter lens to your kit, such as a 50mm f/1.8, you wouldn't have to bump up the ISO or add a flash nearly as often.
The second major impact is how you can manage the background of the composition. A large maximum aperture, such as f/1.8, gives you more options for making the background detailed or soft. Large aperture settings allow you to soften the background more easily, while smaller settings (f/5.6 and smaller) tend to render more background detail.
If your shots tend to have an even amount of detail from front to back, or if they haven't pushed the limits of existing-light photography in moody environments, then you may want to consider a lens with a larger maximum aperture. Reasonable affordable examples are a 50mm f/1.8 and a 30mm f/2.0. One of my favorite affordable "big aperture" lenses is the 85mm f/1.8. With any of these choices, you can shoot without flash indoors and have more control over background detail outdoors.
Size and weight
Maximum aperture also influences size and weight. The larger the aperture opening, the bigger the lens. A great example is to compare professional-quality 70-200mm zooms. A Canon f/2.8 model weighs 3.28 pounds, while the f/4 version is much lighter at 1.67 pounds, about half the weight.
This is also the case for wide-angle zooms and fixed-focal-length lenses. A 17-40mm f/4 wide-angle zoom weighs 1.05 pounds, versus 1.4 pounds for the 16-35mm f/2.8 model. And a Sigma 50mm f/1.4 is considerably heavier than the Canon 50mm f/1.8.
Of course, maximum aperture isn't the only influence on size and weight. Robust pro-model construction also plays a part. That Canon 50mm f/1.8 I mentioned is a consumer-grade lens with plastic construction, whereas the Sigma f/1.4 is a professional model with a metal housing.
How much weight are you willing to carry? That beautiful 70-200mm f/2.8 doesn't do you any good sitting on the shelf at home because it doesn't fit in the shoulder bag you want to carry while touring Paris. It's important to hold the lens you're considering and see how it fits in the bag you want to use.
If you shoot with a Canon, Nikon, or Panasonic camera, then you need an image-stabilized lens to take advantage of this important technology and compensate for camera shake. Those brands use optical stabilization that's built into the lens. Olympus and Pentax, on the other hand, build the stabilization feature into the body, giving you that technology regardless of the lens you've mounted on the camera.
Even though it costs more, I recommend buying stabilized lenses for those models that give you the option, especially for telephotos and long zooms. The professional-caliber Canon 70-200mm f/4 IS zoom costs $600 more than the non-IS model. That's a lot of money! But having image stabilization gives you the opportunity to get sharp shots in a greater variety of lighting conditions.
This final consideration might sound a little touchy-feely, but hear me out. Spending your money on a lens you crave versus one you think you should buy is an important factor. A lens that you bond with--one that is attractive to you, something that you deem amazing--will inspire you to shoot.
Here's a personal example. I recently invested $499 in a Panasonic/Leica 25mm f/1.4 prime lens for my black Olympus OM-D E-M5. I selected that particular glass from a long list of desirable lenses for several reasons.
Maximum aperture: It has a superfast maximum aperture of f/1.4 for low-light shooting and soft backgrounds.
Size and weight: Even though it's relatively heavy for a Micro Four-Thirds lens at 200 grams, it weighs far less than the 505 grams of my Sigma 50mm f/1.4 for my Canon DSLR body.
Emotional appeal: The handsome black finish combined with a large front glass element with Leica inscription pushed my desire meter to the red zone. I simply wanted this lens more than anything else on my list.
Bringing it all together
A new lens is energizing. Photographers tend to like their camera bodies, but love their lenses. When considering your next optic, use these five criteria to narrow the list of possibilities to a handful of favorites. Think about the types of shots you want to capture, the lighting conditions where you might use the lens, and the bulk you're willing to carry.
Once you have your short list of candidates, then read reviews, study the specs, and consider your budget. After you make a final decision, go shoot and make beautiful images.