Some say, that in photography the photographer does the artwork, the camera does the mechanical part, and the lens makes it look good. We’re going to look at the way lenses work and the basics of choosing the best lens for you.
Lenses come with all sorts of numbers and designations. They may seem complicated, but that’s because they were developed by physicists, and in the early days of photography, you had to be part scientist to operate a camera. Let’s try to explain things simply.
There are two designations on a lens that are important. The focal length and the f-stop.
The focal length was originally a measurement of the distance from the lens to the film. When lenses were a lot simpler than they are now, the laws of physics and optics said that a longer lens magnified more. So a 100 mm lens (physicists measure in mm millimeters) made things look twice as close as a 50mm lens. And a 200 mm was four times as close as a 50, and so on.
We’ve messed the measurements up a little bit with newer lens materials that bend light better than glass. So lenses are measured in “effective” focal length rather than actual.
Photography grew up with 35 mm cameras. The 35 is a measurement of the film size, and on a 35 mm camera, a 50 mm lens sees with about the same perspective as a person, so 50 mm was considered standard. A 28 mm lens sees a field about twice as wide as the standard 50, and its called a wide angle. Anything magnified, like 135 mm or 300 mm, are called telephoto.
Here’s the breakdown:
21 mm = super wide angle
28-35 mm = wide angle
50 mm = normal lens
100-300 mm = telephoto
300 mm on up = long telephoto
The necessary focal lengths change with the film or image size. So where a 50mm is normal for a 35 mm camera, 28 mm is normal for some DSLR’s, and 11 mm is normal for many compact cameras. To avoid confusion, most lens manufacturers will list their lenses as “equivalent to” the focal length they would be on a 35mm camera.
F-Stop is the second measurement that’s important when lens shopping. Named after a physicist, Augustin-Jean Fresnel, who invented the lenses used for lighthouses, an f-stop is a measure of how much light a lens lets in. For most photography, the more light, the better, and the way to get more light in is to have a bigger lens. F-stop is just a measurement of size. Here’s an example from Wikipedia:
The important part to remember is that a lens with a lower maximum f-stop will let in more light, take better pictures in low light conditions, and will be larger and more expensive.
So to sum up: Focal length tells us the magnification or zoom, f-stop tells us how much light it can let in.
Now let’s talk about zoom lenses. They do things that seem to break these rules.
Modern technology has created materials that bend and focus light faster than glass. Hence today’s eyeglasses and lenses can be much smaller and lighter than before. And combining different types of lens materials reduces the size required even more. That means today’s lenses, especially zooms, can be shorter than their listed focal lengths.
The tradeoff is that it takes a lot of glass (actually optical glass or plastic) to do this, and there are slight losses anytime light has to go in and out of a lens. The result is that, as a general rule, feed focus (non-zoom) lenses are slightly sharper than zoom lenses. Many photographers feel that the slight loss is an acceptable trade-off for the extra versatility, but the final decision is up to you.
Here are a few places to start shopping for zoom lenses:
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