I have a couple of Kenko Nikon ones. No problems even with 2x on an F4 prime lens. They are nicely made but use plastic of course. The 1.4x is new. The 2x used. After i bought - ebay - the seller asked me if I really wanted it as on his zooms it would only work in AF part through the range and if sufficient light. That and effective F ratio is the problem. I haven't tried it on my 70-300mm. As I see things the main advantage of faster longer lenses F2.8 in particular is the ability to use a converter.
I have seen comment that F8 and phase AF is a no no,
Kenko - another Hoya company as per Tokina. Some of their products such as extension tubes don't really justify the price despite the name.
John
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