The Pentax K-mount, sometimes referred to as the "PK-mount", is a bayonet lens mount standard for mounting interchangeable photographic lenses to 35 mm single-lens reflex (SLR) cameras. It was created by Pentax in 1975, and has since been used by all Pentax 35 mm and digital SLRs and also the MILC Pentax K-01. A number of other manufacturers have also produced many K-mount lenses and K-mount cameras.
Mounts
The Pentax K-mount has undergone a number of evolutions over the years as new functionality has been added. In general, the term K-mount may refer to the original K-mount, or to all its variations.
Originally designed by Zeiss for an alliance with Pentax, it was intended to be a common lens mount for a proposed series of cameras and lenses. However, the plan failed to work out and the two firms parted company amicably, but Pentax retained the lens mount and at least one Zeiss lens design for its own use.
K-mount
The original K-mount is a simple bayonet connection with three tabs. It was introduced with the K series of cameras. The lens is locked into the camera with an approx. 70° clockwise turn (when looking at the front of the camera).
The only linkage with the camera is mechanical and involves the aperture. A slot between two of the bayonet tabs on the lens allows the stop-down coupler from the camera to sense the aperture setting on the lens and adjust the light meter display accordingly. Opposite this is the diaphragm release from the lens which extends into the camera body and holds open the spring-loaded diaphragm of the lens. When setting up a shot this keeps the diaphragm fully open. When the shutter is released, so is this lever. It allows the diaphragm to close to the desired setting while the film is being exposed, and opens it again after the shutter closes.
Both of these linkages are arranged so that they are aligned and spring-loaded by the act of inserting the lens and turning it until it locks.
Bodies equipped with the original K-mount include the K series, the M series except the ME F, and the LX. Lenses that support it include those labelled 'SMC Pentax', 'SMC Pentax-M' and 'SMC Pentax-A'. These K-mount bodies cannot use lenses that lack an aperture ring, such as FAJ or DA.
K-mount lenses can be used on all Pentax bodies, but are restricted to stopped down mode when used with "crippled" KAF-mount bodies (see below).
KF-mount
The KF-mount was Pentax's first attempt at an autofocus system. This autofocus system used sensors in the camera body and a motor in the lens. The two were connected via five new electrical contacts on the bayonet mount itself. One permitted the lens to turn on the camera's metering and focus sensors, two focused the lens (towards and away from infinity) and two appear to have been unused and may have been reserved for future functionality.
The KF-mount was largely a failure. Only one camera and one lens ever used this mount, the Pentax ME F and SMC Pentax-AF 35-70/2.8. The lens was somewhat large and cumbersome since it had to enclose both the focusing motor (with gears) and batteries to power it. KF and the ME-F are similar in many ways to the system used by Canon in the ill-fated Canon T80, introduced several years later.
The ME F can use all Pentax K-mount lenses which feature an aperture ring. The 35–70 mm lens can be used on all other Pentax K-mount bodies in manual focus mode, but it must be used stopped down on "crippled" KAF bodies.
KA-mount
The KA-mount is derived from the original K-mount. It allows the lens's aperture to be set by the body, and thus permits shutter priority and program auto exposure modes. It was introduced in 1983, and is supported by A-series and P-series bodies; Pentax lenses that support it are marked 'SMC Pentax-A'. It is completely backward-compatible with the original K-mount.
The aperture on the lens is set from the body by the same stop-down lever found on the original K-mount, but on KA-lenses this lever is proportional to the area of the aperture opening, rather than the diameter as on previous lenses. This allows the body to easily set a specific aperture, since the relationship to F stops is linear. The lenses add an 'A' setting on the aperture dial, which gives the body control of the aperture. Other, numeric settings are used for manual aperture modes—aperture priority and full manual mode.