What is the meaning of single focal length?

Question

This is a single focal length Nikon lens: http://www.nikon.co.in/products.php?categoryid=1014

What does that mean?

What are its disadvantages as compared to multiple focal length lenses?

Answer

A single focal length, or "prime", lens has a fixed focal length, as opposed to a zoom lens which has a range of focal lengths. So a prime lens will be a 35mm lens, or a 50mm lens, or a 200mm lens. A zoom lens will cover a range, say 70-200mm.

Prime lenses are generally sharper, less expensive, lighter weight and have larger maximum apertures, which is useful in low light. This is because these lenses can have a much simpler design, since the lens only has to work at the one focal length, and can be optimised for it.

Zoom lenses have the advantage that you can use one lens and cover a range of focal lengths, rather than owning several prime lenses and having to swap lenses all the time. You can also zoom in or out to compose a shot. With a prime lens you'd have to move closer to, or farther away from, the subject ("zoom with your feet"), or switch lenses.

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