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Forum: Pentax SLR Lens Discussion
07-17-2018, 08:29 AM
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The aperture lever on the basic K or M lenses only has to open and close the aperture blades. All of the precision was in a slot on the mount and a bar that moved along with the lens's aperture ring. The camera followed that bar. Look at a Pentax K or M lens, and the lever moves in a straight line, because that's exactly how the camera's arm moves. The camera arm was also just open-close.
The KA mount was designed to be compatible but also add camera body aperture control. The new cameras could move the arm in precise increments. Pentax then designed new lenses with aperture levers that moved the blades in precise increments as well. Those levers move in an arc. If the aperture arm moves 0.1mm, all KA lenses should move their blades by the same number of stops.
When the camera is tricked into thinking a lens is a KA lens, it will move the aperture lever 0.1mm, but there is no guarantee that the blades will move by the same number of stops at all. You may get lucky within a certain range, like how -40 degrees is the same in C or F scales. Or the lens might have the right kind of aperture movement, which would still work fine as a basic K mount. But not all lenses will work properly.
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