Regards, Alan Chan http://www.pbase.com/wlachan
The real reason, of course, is so the motor can be sized appropriately for the job at hand. When you use a Pentax AF camera the same motor has to be able to focus everything from the smallest, lightest lens to the heavyweight behemoths like the 600/f4 or 250-600/f5.6. That means you either carry around a heavy motor all the time or you end up with a camera that is slow to focus with the big glass.
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