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.
Three different lenses, many different perspectives.
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.
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