Originally posted by Tim Diss Can someone explain what "conversion to screwdrive" means? If this is advantageous why are the lenses not in this configuration already?
Many thanks.
Well, sometimes the older SDM AF system fails altogether. When this happens, the choice with most of these older SDM lenses is to either have the SDM repaired (with the chance it could fail again) or to have it converted to the old alternative camera body screw-driven AF system, which is noisier. Not all of these older SDM lenses are candidates for this option. Some of the newer lenses having the "SDM" designation are actually of a different design which is not subject to this problem. Other newer AF lenses are either designed with DC motors, the newest PLM system, or still using the old reliable screw-driven technology, and that does not mean they are cheap. All of the Pentax elite "Limited" prime lens series are still made using the old screw-driven AF system. The only Limited lens that is a zoom lens, the DA 20-40mm Ltd, also features a quiet and very good DC AF motor as well as WR construction.
The screw-driven AF technology is a very old camera-body based system instead of the lens having its own AF motor, which came later. A good lens-based AF motor will provide quiet, less distracting but superior functionality, with greater possible AF speed and accuracy. Yet depending on the lens and camera model, many lenses designed with the now ancient camera body based screw-driven AF system are still capable of excellent AF speed and accuracy, albeit with some noise.