It seems that the two main options are the Century 2x and a Sony 2x. I haven't been able to find any info on the Sony, although I saw it mentioned in another post. Can anyone offer any info as to the relative merits of these two items? Also, a link to the Sony website for their high quality extenders would be great, I tried but could not find it. Thanks.

After more searches, I found the Sony site, which lists a 1.7x in 58mm and a 2x in 37mm. Is there a Sony HG 2x in 58mm?

No, Sony just has the 1.7X in 58mm.....By the way the Sony HG 1.7 quality is very good.