I read your reply to the other 70D focus issue. You state you like to shoot your 2.8 lens wide open.You may be having problems because the depth-of-field is too shallow at wide open apertures. http://www.dofmaster.com/dofjs.html This may not be t...
You might try contacting Vivitar's Tech support to see if there is a firmware upgrade.I had some luck with a Tamron firmware upgrade for my 70-300 VC and the 7D2. Good Luck!
