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08-09-2008, 08:00 AM   #1
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Does anyone know................

........why the correct Tamron 1.4 and the right Kenko 1.5 TCs support USM and others don't? I speculate that this is accidental in that they were both discontinued around or even prior to the intro of the newer PTX Da*s but would appreciate any technical insights into why these units are 'special'.

Just wonderin'.............

08-09-2008, 10:10 AM   #2
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Maybe because they don't have the two electric contacts that allow SDM focusing?
08-09-2008, 11:04 AM   #3
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Well, I have a Promaster 1.7 AF with the two insode contacts. It will actuate the USM but not achieve focus. I think some Kenkos have the contacts too, but will not work on USM. I have concluded it must be on the electronics, but have no idea why.........
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