Features and Operation of the KAF2 Mount

Features and Operation of the KAF2 Mount

Lenses:
SMC Pentax-FA
Bodies:
Z-series, PZ-series, MZ-S, MZ-3, MZ-5n/ZX-5n, MZ-5/ZX-5, MZ-10/ZX-10
Compatibility Issues:
Bodies in this group are generally usable with the FAJ lenses only in Tv and P operating modes. Bodies that can control the aperture are also usable in M and Av mode. Lenses in this group, with the aperture ring away from to the “A” setting are are virtually unusable with bodies featuring the “crippled” KAF mount. There are no limitations when the aperture ring is set to “A”.

Overview

This mount variation adds power zooming and MTF transferral to the already rich set of KAF mount features.

Power Zooming

At the time of the initial introduction of the KAF2 mount, power zooming was a standard feature implemented in all bodies and all zoom lenses. However the feature is losing popularity, and while newer bodies typically implement only the the most basic functions, most new lenses do not support it at all.

In power zoom mode, the body selects, activates and controls the operation of the power zoom functions. The lens contains the zoom motor as well as a push-button and a three-position slide switch. The zoom ring can be pulled away from the body to enable power zooming or towards the body to select manual zooming. In the power zoom position, twisting the zoom ring changes the focal length at one of three speeds (more twist results in a higher zooming speed).

The following functionality is possible (not all KAF2 bodies support all functions):

  • Auto Lens Retraction: when the main switch is moved to the “OFF” position, the body zooms the lens to the focal length at which the lens has the smallest physical dimensions.
  • Image-Size Tracking: the lens is zoomed in or out in order to maintain constant image size. This is useful when the subject changes its distance from the camera.
  • Power Zoom: the focal length of the lens is changed either via a small twist of the zoom ring or via controls located on the body.
  • Zoom Clip: zooms to the preset focal length (or lengths) when the control button is pushed. The desired focal length is set by adjusting the zoom ring (at the “AS” setting).
  • Zoom Effect: halfway through the exposure the lens zooms in or out, creating zoom lines towards or away from the main subject.

MTF Transferral

In addition to all the lens parameters that are transferred through the digital lens mount contact in the KAF mount, KAF2 adds the modular transfer function (MTF). This allows the body to use apertures at which the lens offers higher resolution.

Operation

Diagram of the Kaf2 mount

Figure 1. Diagram of the KAF2-mount.

KAF2 lenses without power zooming feature a 4-bit CPU that is virtually identical to that used by the KAF lenses, but it supports also MTF transferral. Lenses with power-zooming feature an 8-bit CPU that additionally supports the power-zooming functionality. The body’s central processing unit (CPU) and data processing unit (DPU) interface with the lens’ CPU are responsible for controlling the digital communication.

The power for the zooming motor is supplied over the two additional power contacts which are part of the KAF2 mount.