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Lenses are the eyes through which your camera sees the world. You can change the way your camera sees simply by changing lenses or by changing your zoom setting. Matching the right lens to the right subject is one of the ultimate keys to creative photography. Knowing which lens or zoom setting to use is largely a matter of how you want the subject to look combined with your understanding of the basic characteristics of different types of lenses.
All lenses are described in two ways, by focal length (as measured in millimeters) and by speed (jargon for their maximum aperture size). A 300mm f/2.8 lens, for example, has a focal length of 300mm and a speed of f/2.8. The focal length of a lens is important because it gives you an idea of how its angle-of-view and perspective relate to our own vision of the world. Lenses of about 50mm, for instance, are considered normal because they provide approximately the same angle-of-view and perspective that our eyes see. Lenses shorter than 50mm are considered wide-angle lenses; longer lenses (i.e., those with a greater focal length) are considered telephoto lenses.
You should be aware that lenses for digital cameras are often referred to in their "35mm equivalent" focal lengths rather than their actual physical focal lengths. This is simply because manufacturers know that most photographers used the focal lengths of lenses for 35mm cameras as their point of reference. Also, because the size of the sensor in different digital cameras affects the actual focal length, and because sensor size varies (there is no standard size), it is simpler to use lenses for 35mm cameras as a standard. For these reasons, you will often see the 4X zoom range of a digital point-and-shoot camera referred to as "35-140mm (35mm equivalent)" rather than its actual focal length.
Also, if you own a DSLR, your camera may incorporate what's known as a "cropping factor" in determining the actual focal length. Because the sensor in many digital cameras is smaller than a 35mm frame of film, the "working" focal length of any lens used on that camera increases. If your camera has a cropping factor of 1.5X, for example, the working focal length of that lens is 1.5X its stated size. In other words, if you were using a 100mm lens from a 35mm camera on your DSLR body, and the camera had a cropping factor of 1.5X, the actual working focal length would be 150mm. Your manual will explain this in greater detail.
Lens speed indicates how bright the image in the viewfinder will be. Lenses with larger maximum apertures provide a brighter viewfinder image but are not necessarily sharper or better than slower lenses. The main advantage of faster lenses is that they provide better viewing in dimly lighted situations. For example, a 200mm f/2.8 lens (considered very fast for a lens that long) would produce a brighter viewing image than a 200mm f/5.6 lens. As a rule, lenses with wider maximum apertures are more expensive.Next: "Exposure Basics"