If you need a wide-angle or telephoto lens, but can’t justify the cost, one of these add-on lenses might be just what you need. An add-on lens is attached to the front of an existing lens, and decreases or increases the angle of view of that lens by a certain percentage. Just multiply the focal length of the lens you’re using by the factor that’s in the name of the lens (0.25x; 1.7x) to find the approximate focal length you’ll be seeing when using the add-on. A 100mm lens with a 0.25x multiplier would be equivalent to a 25mm lens. The same 100mm lens with a 1.7x adapter would be equivalent to a 170mm lens.