Originally posted by Steven Stahl I would like to buy an adapter to use the Vivitar FD mount lens on the KA mount Pentax.
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Short answer #1 is that the lens flange distance for FD mount is shorter than that for K-mount, meaning that you would lose infinity focus unless an auxiliary glass element is part of the adapter (reduces optical performance). However, a glass-less version would extend the macro magnification.
Short answer #2 is that there are no adapters I am aware of that allow other than full manual aperture (always stopped down) with FD lenses on K-mount.
Steve