Pentax K-mount

Pentax K1000 without lens, showing the original K mount

Pentax MZ-3 showing the K_AF2 mount, with electrical contacts and autofocus transmission coupling

The Pentax K mount is a lens mount standard for mounting interchangeable photographic lenses to 35 mm single-lens reflex (SLR) cameras. It was created by Pentax in 1976, and has been used by all 35 mm and digital Pentax SLRs since. A number of other manufacturers have also produced K mount lenses, and in a few cases, K mount cameras.

Mounts

The K mount has undergone a number of evolutions over the years as new functionality has been added.

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 of course 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' and 'SMC Pentax-M'. The bodies cannot use FAJ lenses since these lack an aperture ring. The lenses can be used on all Pentax bodies but can only be used in stopped down mode with "crippled" K_AF mount bodies.

K_F mount

The K_F mount was Pentax's first attempt at an autofocus system. It was used by only the Pentax ME F and its accompanying SMC Pentax-AF 35-70/2.8 lens. 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 focussed the lens (towards and away from infinity) and two appear to have been unused and may have been reserved for future functionality.

The ME F can use all Pentax K-mount lenses except for FAJ lenses, which lack 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" K_AF bodies.

K_A mount

The K_A mount is derived from the original K mount. It allows the lens' aperture to be set by the body, and thus permits shutter priority and program autoexposure 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 K_A 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.

Six electrical contacts are added to the bayonet ring. One is slightly recessed and allows the lens to communicate whether the aperture setting is set at 'A'. If it is, a pin on the lens extends slightly and makes contact, while if the lens is at any other setting the pin is retracted out of contact. The other five contacts are used to encode the lens' aperture range. The matching contacts on the lens are either conducting or non-conducting, making the setting binary 1 or 0, respectively. Two contacts encode the lens' minimum aperture—f/16, f/22, f/32 or f/45, although no K-mount lenses have ever had an f/16 minimum aperture. The other three encode the lens' maximum aperture; their meaning is different for each encoding of minimum aperture.

K_AF mount

The K_AF mount was Pentax's second and much improved attempt at adding auto-focus to lenses. It adds a small drive shaft to the K_A mount, allowing the body to adjust the focus of the lens. This makes the lenses less bulky than the earlier K_F mount, which had both a motor and batteries inside the lens.

It also adds a seventh electrical contact, this one carrying digital information from the lens to the camera. It carries the following information: focal length, distance to the subject, exact absolute f-stop value, and lens size. This information is used to make better exposure decisions, along with the multi-segmented metering that was introduced in cameras using the K_AF mount.

K_AF2 mount

The K_AF2 mount adds to the K_AF mount two extra contacts to the inside of the mounting ring for power zooming, and transmits modular transfer function (MTF) data through the digital seventh contact.

K_A2 mount

The K_A2 is identical to K_AF, but lacks the autofocus drive shaft. Thus it only adds the seventh contact for digital information to the K_A mount.

"Crippled" K_AF mount

The "crippled" K_AF mount is used on the MZ-30/ZX-30, MZ-50/ZX-50, MZ-60/ZX-60, and *ist cameras. The only difference between the regular K_AF mount and the crippled version is the removal of the stop-down coupler/indicator.

More information

Bojidar Dimitrov's Pentax K-Mount Page

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