I have a recurring problem with my Canon 5D3. Several times per year I do a series of editorial style corporate location portraits. These are all shot with a Canon 85mm F1.8 lens at full aperture to throw the background out of focus. Exposure varies from 1/80 to 1/125 at ISO 400-800 with the camera hand held, always horizontal.
When I first bought the camera it back focussed with the 85mm and I adjusted it so it was dead on. My technique for these photos is to choose the most appropriate focus point ( in one shot mode ) in order to minimize reframing after focussing on an eye. I lock focus with the shutter release button before reframing, and refocus after each exposure.
Most of the time focus is dead on, however in about 10-15% of frames slight back focus occurs, usually about half way to the ear, sometimes a bit more ( front focus never occurs ). Back focus usually occurs in only one frame at a time, but occasionally in two or three sequential frames, all of this without any repositioning of subject or photographer.
Does anybody have an idea as to what may be causing this? Light levels are good, and an eye provides good contrast to focus on so I am not sure what the cause might be.
