Originally posted by johnha 
Umm, Pentax did with the 1.7x AF adapter...
Just in case you're not kidding here, the 1.7X AF is a teleconverter with glass elements and not just a simple adapter like the M645 to K item. Teleconverters combine with the lens they're attached to resulting in a new lens configuration with a different focal length. Similar to above, each lens made has a focal length because it's a measured optical property of the design used. Focal length does not change when its circle of illumination is cropped by adapting it to a smaller format.
Post #3 by Ontarian50
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