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. Six electrical contacts are added to the bayonet ring. One is slightly recessed and allows the lens to indicate whether the aperture ring is set at 'A' or not. If it is, a pin on the lens extends slightly and makes contact, while if