Supersonic Drive Motor (SDM)
The Pentax-developed SDM, with SDM standing for "Supersonic Drive Motor", departs from the previous Pentax system which utilizes a screw-drive autofocus motor inside the camera body. Instead, SDM lenses incorporate an autofocus motor as part of the lens itself. All current production Pentax SLR bodies are compatible with SDM lenses. However the DA* 55mm, DA 17-70mm and DA 18-135mm lenses are not compabile with some older Pentax digital SLR's such as the K110D, first version K100D and earlier models (The K100D Super is compatible). .
The SDM mechanism is claimed to provide smoother and quieter focusing. The first Pentax cameras to support SDM lens were the K10D (firmware 1.30 or later) and K100D Super. These and subsequent Pentax DSLR bodies support both SDM and in-camera screw-driven AF.
Read more about this topic: Pentax Lenses
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