Autofocus Variations Can Ruin Your Focus Accuracy Whilst the autofocus has been improved with successive generations of the camera is still relies upon contrast detection to set the focus point. So the camera moves the lens to the point of maximum contrast and then steps forwards and backwards to get the precise point. Because of mechanical tolerances in the focus mechanism sometimes the repeatability of the focus point is questionable. I noticed some of my shots were not exactly sharp at the pinpoint area that I knew I had set the camera to when I took the shot. So I set up my DSLR focus chart used to correct the optical viewfinder to sensor calibration and shot a series of 10 images in AF single area and smallest target size. There was a noticeable shift in a lot of the images as you can see from the resulting images. Compare that to the same target where I manually set the focus point.

Link to the original article