A problem that can occur for the first time consumer of lenses for a DSLR type camera is the the issue of photo sensor dimensions. If you did not purchased a DSLR with a "full frame" (approx. 35mm) sensor, the focal length printed on your lenses will be inaccurate.
All consumer and many enthusiast DSLR's are equipped with a photo sensor smaller than the traditional area that 35mm film uses to capture an image. All lenses that I know of, use the 35mm equivalent to describe the focal length of the lens.
Read your owners manual to determine what "crop factor" your individual camera functions at compared to the equivalent 35mm sensor area. Many DSLR's run at 1.5X, 1.6X 1.7X and 2.0X as multiples of the advertised focal length of a particular lens.