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What do the numbers mean on a camera lens?

Lens Basics


Lenses don't have fun names, but rather are labeled like this: 18-55mm f/3.5-5.6. That designation tells you a lot, but doesn't mean much if you don't know a few terms. 


IS = image stabilization - an electro mechanical system in the lens that counteracts camera shake.

USM = Ultrasonic Motor - refers to the focusing motor in the lens. USM lenses focus faster.






Zoom Lenses vs. Prime Lenses

Zooming Lens
 A zoom lens does what you might expect: it zooms in and out, allowing you to magnify your subject. A prime lens can't zoom. If you want to move closer or farther away with a prime, you move your legs rather than turning a zoom ring on the barrel (i.e. the body) of the lens. Why would anyone prefer a prime to a zoom? Primes are often cheaper, offer sharper results, have wider apertures (which means they can take in more light, thus performing better in low light situations), and generally handle their specific function better than a zoom. That said, for enough money a zoom can perform just as well and offers more versatility. Obviously you have an advantage if you don't have to

Prime Lense
constantly change lenses. Zooms allow you to essentially change the lens, and therefore the type of shot you're getting, by zooming in or out. Primes do not.
 
Both zoom and prime lenses are designated by their focal length. Focal length is often measured in millimeters (mm) and designates the distance at which something is in focus. This definition doesn't mean much practically speaking, so what you want to remember about focal distance is that lower numbers indicate a wider view (zoomed out) and larger numbers indicate a closer view (zoomed in). 
For example, if you're photographing a small room in a house and you used an 18mm lens you'd likely capture the majority of the room in your image and you'd get very little of the room with an 85mm lens. Primes only have one focal length, and so they're simply labeled as 35mm, 50mm, and so on. Zooms offer a range, and so they're labeled as 18-55, indicating that you can achieve a focal length as wide as 18mm, as close as 55mm, and everything in between.

Aperture


Lens titles have two measurements. The first is focal length, as we just discussed, and the second is aperture. Aperture determines how much light your lens can let in. When a lens has a wide aperture, designated by a small number called an f-stop (e.g. f/1.8), it captures more light. When a lens has a narrow aperture, designated by a larger number (e.g. f/5.6), it captures less. Wide apertures allow you to take photos when less light is available because the lens can see more of the light. Take a look at the photo to the right. The pictured lens current has a wide aperture, and you can see a lot of light passing through the front glass of the lens straight through the back. If that hole were smaller, the aperture would be narrower and less light could pass through. Aperture isn't just a concept, but something you can actually see when you look at a lens.
You're not stuck with a single aperture. When a lens is rated a nice, wide aperture like f/1.8 that just designates its maximum. You can change the aperture on your camera to a narrower aperture to allow less light. Why would you do this? Narrower apertures provide a greater depth of field, meaning that more of the image will be in focus. When you're photographing a landscape, you want the entire image to be in focus and not just the part of the plane that your camera focused on. A higher, narrower aperture (e.g. f/12) provides that. A wider aperture (e.g. f/2.8) would make the landscape appear less sharp. That said, you don't always want everything in focus. When taking a portrait, for example, wide apertures are wonderful because you can focus on a person and allow the background to blur away. As demonstrated in the example photo to the above right, the only things in focus are the aperture blades of the lens. A wider aperture made this possible.











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