Original sentence: For that reason, when using the manual aperture ring on K-mount lenses, it is necessary to use stop-down metering mode to ensure proper exposure.KA and KAF lenses with aperture dials are fully compatible when the aperture dial is set to automatic ('A'), giving control over the aperture to the camera body.Like the KAF mount used on previous Pentax digital SLRs, this lacks the mechanism to read the aperture setting set on the lens' aperture ring.The K10D is the first Pentax camera to feature a KAF2 mount which drops support for on-lens manual aperture settings.This choice trades off backward-compatibility with the oldest manual lenses for the assurance that a compatible lens will always act in accordance with the chosen exposure mode (e.full-auto/aperture priority/shutter priority)
Edited sentence: Like the KAF mount used on previous Pentax digital SLRs, this lacks the mechanism to read the aperture setting set on the lens' aperture ring and is thus only fully compatible with K mount lenses that have an 'A' auto setting on the aperture ring, or no aperture ring at all.Older lenses can only be metered at their widest aperture or in stop-down metering mode, and operate only in aperture-priority or full manual settings.The K10D is the first Pentax camera to feature the  KAF2 mount