High power zoom lenses use Internal FocusIF) systems to shorten the Minimum Focus Distance. When an IF system is used in the optical system of a lens with a wide to tele range, the magnification ratio at telephoto is shortened when you shoot closer to the MFD. When pictures are taken at a close focusing distance with a zoom lens at its telephoto end, the zoom lens covers a wider angle of view than would a fixed telephoto lens. However, since the focal length of any lens is based on the focusing distance at the infinity setting (and Tamron’s high power zoom lenses provide the same angles of view as other telephoto lenses at the infinity setting), this phenomenon is not a defect or flaw in a zoom lens.