What is the meaning of single focal length?

Question

The Nikon AF-S DX Nikkor 35mm f/1.8G is described as a single focal length 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 1

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.

Answer 2

A single focal length lens is often called a prime lens. There are only really two key types of lenses from a focal-range standpoint: primes and zooms. In most cases, a prime lens is optically superior to a zoom lens of similar focal lengths, and tend to have larger maximum apertures. This is because prime lenses can be built simpler, usually with fewer elements, and all correction for any optical aberrations can be maximized in potential for the single focal length the lens represents.

On the flip side, they limit your composition unless you physically move the camera. Zoom lenses, on the other hand, do not have that limitation, and they allow changes to composition within the bounds of their focal range without requiring you to move the camera. Zoom lenses tend to be more complex than primes, particularly if they straddle the flange distance of the camera (the distance from the lens mount to the sensor/film). Zooms usually have additional moving element groups to support changes to focal length beyond the focusing group, which limits the ability to correct optical aberrations. Additionally, optical aberrations must be corrected such that they are as good as they can be at all focal lengths of the lens, not just a single focal length.

Lenses, primes included, that have a focal length shorter than the flange distance usually require some kind of additional correction to adjust the focal depth such that the virtual image projects properly onto the imaging surface. This is done with inverted telephoto element group at the back of the lens, leading to retrofocal design. This allows ultra wide angle lenses to be used on cameras with relatively large flange distances, which tends to be the case with SLR type cameras. As such, normal focal length lenses, 50mm-85mm or so, will usually exhibit some of the best quality optics at reasonable prices.

Answer 3

(Very) simply put, it means that the lens cannot zoom in or out.