Features and Operation of the KAF Mount

Features and Operation of the KAF Mount

Lenses:
SMC Pentax-F
Bodies:
SF-series, MZ-6/ZX-L, MZ-7/ZX-7
Compatibility Issues:
The SF are usable with the FAJ lenses only in Tv and P operating modes. MZ-6/ZX-L, MZ-7/ZX-7 are able to control the aperture from the body, so they are fully compatible with the FAJ lenses. The F-series lenses, with the aperture ring away from to the “A” setting, are virtually unusable with bodies featuring the “crippled” KAF mount, but there are no limitations when the aperture ring is set to “A”.

Overview

The KAF mount extends the KA mount by adding two new features. The less obvious of the two is the addition of a seventh body-lens contact, next to the six that were already added by the the KA mount. The new contact is used for exchanging digital information, and in the KAF mount supports the exchange of the following lens paramters: focal length, lens-to-subject-distance, exact absolute value of the selected f-stop (even for variable-aperture zooms), and lens size. This information is mainly used for making flash operation easier and less error-prone. For example, the lens physical size is needed so the body can decide if the lens will interfere with the use of its built-in flash.

The other addition to the KAF mount is auto-focus (AF). The only visible sign of the AF capability is a shaft coupling which allows a motor contained inside the camera body to focus a KAF or KAF2 lens i.

The KAF mount is AF-incompatible with the KF mount: since the KF lens does not have a shaft receptor, the KAF bodies cannot focus it and since the KAF and KAF2 lenses do not have built-in focusing motors, the KF body cannot focus them.

Knowing that AF does not solve all focusing problems, the Pentax engineers added a way to turn AF off. They also were very careful to design the new mount so that it remains fully compatible with the older manual focus ones. To aid the operation with manual focus lenses, they carried over the “focus confirm” function from the KF mount and added a new feature — focus lock. When the focus lock is enabled, pressing the shutter release does not result in an exposure until the body detects proper focus.

The AF “era” allowed the advance of new-generation focusing screens. In place of the grainy screens that featured focusing aids and were aimed at making manual focusing easier, now appeared brighter and less contrasty focusing screens without focusing aids. While the new screens make composing an image easier, achieving precise focus with a manual focus lens becomes quite a challenge.

AF has some negative sides too. For example, since the AF motor uses battery power for focusing the lens, the focusing mechanisms of the AF lenses are designed in such a way as to provide extremely low resistance. As a result, the manual focus feel is quite light and somewhat “wobbly.” Furthermore, most AF zoom lenses also feature rotating front elements which makes the use of polarizing and other special-effect filters less convenient.

Although multi-segment metering was already possible with the KA mount and KA lenses, the first bodies to implement it were the KAF ones. In this metering mode the body gathers exposure information from a number of areas in the frame, and thus is able to compensate for back-lit as well as other difficult metering situations. As a result, “proper” exposure is achieved more often. On the other hand, the algorithm is rather complex, and it cannot easily be said when it will fail and in which “direction.” Hence, using exposure compensation together with multi-segment metering for one single frame does not make much sense. Bracketing the important shots is still a very good idea.

Operation

Diagram of the Kaf-mount
Figure 1. Diagram of the KAF-mount.

Digital Communication

The KAF mount features the same six electric contacts as the KA, and thus remains fully compatible with the older mounts. However, in addition to those (electric) contacts, it features a seventh — electronic one. Using a yet-unknown serial protocol, the body uses this contact to communicate with a digital chip contained inside the lens housing. The chip provides at least the following information:

  • Focal length of the lens. This allows more efficient flash operation as well as calculating the longest hand-holdable shutter speed. In addition, it is also one of the parameters taken into account when the body is in one of the “smart” picture modes and has to choose a program curve. Prime lenses provide one fixed value, while zooms provide an approximate value that changes as the focal length changes. The focal length information is precise enough to distinguish between 35, 28 and 24 mm lenses, as evidenced by F-flashes that trigger a warning when they cannot cover the field of view of the lens.
  • Lens-to-subject distance. This is used by the more advanced bodies for calculating if the flash is powerful enough to illuminate the subject that is in focus. In addition, this is one of the parameters taken into account when the body is in one of the “smart” picture modes and has to choose a program curve.
  • Exact selected aperture. The KA mount supports displaying the picture-taking aperture when the “A” aperture setting is engaged. The KAF mount supports displaying the aperture even when a numeric f-stop is selected. The displayed value is quantized by half-stops, but is correct even when a variable-aperture zoom lens is used.
  • Lens dimensions. The lens also gives the body enough
    information for determining if the lens is going to cast a shadow on the subject when the camera’s built-in flash is used. This parameter takes into account the size changes due to zooming and focusing too.

The lenses contain a ROM chip that holds the program curve (brightness/shutter speed/aperture) for prime lenses or up to three program curves for zooms at their different focal length settings. The distance information is hardwired on the lens barrel. A number of tiny parallel conducting/non-conducting strips are glued to the rotating part of the lens. The same number of contacts are attached to the fixed part of the lens. As the lens barrel turns during focusing, different combinations of the strips are detected by the contacts. There are four parallel strips in the F 50/1.7, resulting in at most 16 different distance settings. But for that particular lens not all combinations are used.

A similar technique must be used for the focal length.

Multi-Segment Metering

This metering mode operates only with A-series and later lenses. A good question then is “why.” John Francis suggests that the body needs to know the lens maximum aperture so it can make an absolute measurement of the lighting. Without such knowledge (and this is what is not possible with K and M lenses), no distinction can be made between an f/1.4 lens on an overcast day and an f/4 lens on a brightly-lit snow field.

The next question is then “how can the body distinguish between A-series and earlier lenses (even when the aperture is off the “A” setting)”. Some Pentax literature points out that no matrix metering is possible with the A 50/1.2 lens. This lens is the only one whose mount contacts are all metal, i.e. its mount is identical to that of a K/M series lens. All other A lenses have at least one plastic contact. So the bodies must be looking for at least one non-conducting mount contact.

i = Further details on the AF operation are contained in the AF description page.