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The World Through a Lens
It's often said that lenses are the eyes of the camera, allowing it to see up close, far away, and all points in between. You can create lots of good images with just one lens, but why stop there? There is a whole world of camera lenses that go far beyond the basics, making it possible to capture astonishing images and allowing for far greater creativity.
Learning About Lenses
A lens in its simplest form is a pretty basic thing: a piece of transparent material, usually glass, with two polished surfaces that modify rays of light. Before the advent of SLR photography, most cameras had built-in lenses that captured images similar to what people saw when they looked through the viewfinder. Such things as making far-away images seem closer or larger wasn't possible with these lenses.
The ability to make images larger and smaller and to have greater control over such aspects as depth of field, angle of view, and perspective is what made SLR cameras popular when they were first introduced, and it continues to be a key factor in why more photographers use them than any other camera.
Today, there are four basic types of lenses for SLR cameras:
Normal, or standard, which is similar to what your eyes can see.
Telephoto, which makes far-away images appear closer.
Wide-angle, which covers a wider angle of view than is possible with a normal or telephoto lens.
Zoom, which combines the features of the first three types into one lens.
Focusing on Focal Length
What distinguishes each type of lens is a factor called focal length. Expressed in millimeters, such as 50mm, 100mm, and so on, focal length describes the distance from the optical center of the lens to the film. It also determines the scale of the images in a picture. In general, lenses with short focal lengths make images look smaller. As focal lengths get longer, image sizes get bigger. If you were to stand in the same place and take a picture using lenses with different focal lengths, you would get the widest angle of view and the smallest images with the shortest focal length lens. As you went up in focal length, images in the scene will look bigger, but you'll see less of the scene itself as the angle of view narrows.