Originally posted by selar I don't think the motor will be damaged. All the kenko is doing is providing the electrical contacts. My sigma 70_200 hsm hunts a bit at the long end but its fine with the kenko till about 150. I suspect its more due to light not being sufficient for auto focus than an electrical issue as it was nearly sunset when I had the hunting issue. Probably same with da300 as its only f4.
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I don't think it was the light affecting my DA300mm and the kenko but the fact the camera probably didn't know the proper distance information. I got the same effect with a TC on a 40D canon body when taping the pins on the TC to get the camera to attempt to focus as non 1 series canon bodies won't even try to AF at less than f5.6.
I wonder if Pentax couldn't find a fix either as the TC they were promising a few years ago (even had a prototype at a show) never went into production. Just a hunch.