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04-01-2012, 06:22 AM   #31
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The Pentax adapter really only works correctly with lenses that have a base large enough to reach the K lens mount surface, which locates the lenses relative to the "film plane." The adapter is pulled slightly below the mount surface by the tension springs in the mount, so that it pulls the lens tight to the mount surface.
Some smaller lenses, like the Asahiflex with M37-42 adapter, dont reach the K mount surface, and so the adapter stays loose within the mount, and the lens doesn't solidly locate at the correct flange-sensor plane distance.
All modern bayonet mounts use springs to pull the lens against the flange face, so they don't loosen with time. I have an old Exakta that was made before they added springs for that purpose, and indeed the lenses are loose. The other one I have was made a few years later, and added springs to keep the mount tight.
That used to be the argument for screw-mount lenses: that they always mounted tightly, but the modern spring-loaded bayonet solved that problem. But a small-base lens that won't reach the mount "locating" surface will not work properly even with the adapter.

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