If you want to increase or magnify the focal length of your lens you could use a teleconverter, which is also known as a tele-extender and sometimes as two words, tele converter. A teleconverter is a small lens that is mounted between a camera body and a lens to increase the focal length of the lens. Different teleconverters have varying degrees of magnifying powers. They generally range from 1.4 times to three times the normal magnifying power of the lens.
The purpose of the teleconverter is to increase the focal length of the lens without having to change lenses or buy a more expensive lens. A 3X teleconverter attached to a 100mm lens will triple the image size by tripling the lens's focal length to the equivalent of a 300 lens. Only use teleconverters when necessary, as you may notice some light loss and a lower quality in the image. Teleconverters range in price from about $50 to several thousand.