M42
Pentax supplies adapters to fit M42 screw-mount lenses, as do several third-party manufacturers. The M42 screw-mount system was used by Pentax prior to the introduction of the K mount. Pentax designed the K-mount wide enough to allow an adapter to fit between the M42 thread and the K bayonet. They also kept the same flange focal distance (also called registration distance or register) as the M42 screw mount, so that M42 lenses focus correctly using the correct adapter (such as Pentax original or Bower). There are however other third-party adapters that add to the flange focal distance so that one loses the ability to focus to infinity. The loss of infinity-focus may not be significant in macro or close-up photography.
There is great debate in the Pentax community over the applicability and safety of adapters other than those supplied by Pentax. Many users of third-party infinity-focus adapters, such as Bowers, report difficulty in removing the adapters from camera bodies. Such adapters may require modification before they may be safely used. Official Pentax adapters, and flanged non-infinity-focus adapters, do not provoke such problems.
Many old M42 lenses have a modern-day cult reputation, including the (Pentax) Asahi Takumar range. Some manufacturers, including Carl Zeiss AG, still make lenses in the M42 mount. K-mount cameras have a suitable flange focal distance (45.46 mm) to adapt old M42 lenses without any optical correction or loss of infinity focus/changed close focus distance. Other SLRs with a short flange-focal distance can accept M42 lenses as well: Canon E-mount (44.00 mm), Sony and (Konica) Minolta A-mount (44.50 mm), Sigma (44 mm), Olympus 4/3rd (38.67 mm), and many more, but notably not Nikon F-mount (46.5 mm).
Read more about this topic: Pentax K Mount, Adaptors To Use On K-mount