The type of lens fitting varies between different brands of camera. For example it is not possible to put an Olympus lens on a Pentax body, or vice versa. If buying a new lens make sure it is the correct fitting for your camera. If you already own a manual focus camera and trade-up to an autofocus SLR, you will usually need to invest in new lenses, too. Only Nikon and Pentax have kept compatibility (and even then the lenses will remain manual focus and not work in all exposure modes).