auto focusing

My Tamron 18-200mm VC lens doesn't compare well in terms of autofocus with another lens I've got. I wanted to see how sharp the lens is at 200mm, so I did some tests. With autofocus (single point) on a small target, about 8 out of 10 shots are reasonably sharp. The other 2 are slightly off ie...

Hello, I have recently purchased a used Sony NEX-7 and and have been using it with my Sony 50mm f1.8 OSS E Mount, SEL50F18 When the camera is focusing using Flexible spot AF or centre point the AF is pulsing slightly backwards and forwards on the target, meaning I cannot get a sharp image ! ...