Pentax bayonet mounts

The *ist press release states that the new SLR features KAF mount. That's not correct, since it lacks diaphragm simulator. Since so many variations of the K bayonet do exist, let's start with a quick survey of Pentax bayonet mount evolution (see figure below). 

The original K bayonet (1975) was of completely mechanical type, featuring a diaphragm simulator coupling (4) for full-aperture metering, a camera-to-lens stop-down actuator (2) for when taking the picture (actual diaphragm set on aperture ring) and lens release mechanism (3). In this operation, the stop-down actuator simply releases the diaphragm, while actual taking aperture is manually set by the lens ring. This allows both aperture priority and manual exposure.

Leaving aside Kf mount for the ME-F (which had no further development), the most important improvement came with the Super A (1983). In order to allow shutter priority and program exposure, six electrical contacts (c) were added, for transmitting minimum aperture and aperture range of each lens (coded, in half stops) to the camera. Contacts r1, r2 and r3 transmit diaphragm range, while m1 and m2 tell minimum aperture (f/16, f/22, f/32 or f/45). Provided that the lens diaphragm ring is set to A (auto), a ball contact (*) is raised on the lens bayonet. This way the camera is informed that it has to read at full aperture (as usual) and then operate the diaphragm, by means of a proportional movement of the stop-down actuator. To that purpose, when changing K bayonet to KA type, Pentax modified the way the stop-down mechanism works in their lenses, from incremental to linear (each f/stop needs the same angle movement). This improved bayonet also allows both shutter priority and program exposure.

In 1987, with the introduction of the autofocus SF-series cameras and F-series lenses, an AF coupling (5) was added between body and lens. A seventh contact (d) was of digital type, transmitting information about the lens by using proprietary serial protocol. Of course, KAF mount also retained all of the features of the KA bayonet. 

Then, in 1991, the Z-series SLR's introduced the improved KAF2 mount, featuring power zoom and new, more refined, electronics for digital transmission. A new ROM inside each lens tells the camera more information and also causes some incompatibility problems with third party lenses and their (not fully) compatible electronics. Two more contacts (6) were also added to the mount for powering the zoom motor in each lens. 

For better understanding, here is a schematic of KAF2 mount, the most advanced Pentax bayonet so far, and its working parts.

For further in-deep info about K mount development, variations and compatibility, visit http://www.bdimitrov.de/kmp/

In spite of so much electronics added during the years, all of the mechanics was still there to grant full backward compatibility. That's the Pentax way, one of the best reasons for sticking to Pentax, you know.

Canon, Contax and Minolta changed their lens mounts without caring so much of their customers, Nikon did "improvements" which leave their cameras meterless when using older lenses (and most people will continue thinking that current SLR's still use the original F mount). What about Pentax? Sure Pentax won't do that to their loyal customers...

Unfortunately, some certainties start to fade. Starting with the MZ-50 (1997), Pentax entry-level SLR's (MZ-50, MZ-30 and MZ-60) started using the so called "crippled KAF" mount. This unoffocial designation has been dubbed by Pentax aficionados, for distinguish it from the full-working KAF. The crippled KAF features no diaphragm simulator, hence compatibility with K- and M-series lenses (the most diffused ones in secondhand market) has gone. Here follows a chart with features of different Pentax K mount variations as seen so far.