I used my new Sony 70-200/f4 on a weekend trip recently and took a lot of street photos using this lens. Looks OK on screen and also at home, until I zoom in. Then most pictures are a little bit un-sharp, like if it isn't really focused. I used f-values between 4-7.1 and shutter speed of at least 1/400, so I don't think movement is a problem. And the edges are not fuzzy, it is more like you can't see the smallest details, like pores in the skin etc. Is my lens in need of calibration of what can the problem be? I'm very new to this kind of tele-lenses, so I don't know. I use AF single with medium spot btw and aim for anywhere on the face.