I'm using a friends Kenko Teleplus Pro300 2x convertor for the moment, though not with either of the lenses you mentioned.
It's been used with my Canon 75-300 USM
III non IS
lens. As with all 2x extenders it reduces the aperture at 300mm from f5.6 to f8, which then means that the AF
won't function. There is a way to fool the system by taping some of the pin connections between the lens and the convertor but I don't recall ever getting that to work.
I've not been able to take any decent handheld shots with the doubler on. What I have found it useful for is taking shots of the moon, but this means setting up on a tripod, setting mirror lockup, manual focus and timer shutter release. Even then the results are very hit and miss but that's more likely down to my ineptitude at manual focusing!
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