Often soft and blurred images are mistaken as user focusing errors or camera shake. Steps should be taken to eliminate these possibilities as in selection of a manual focus point or group, and ensuring the shutter speed matches the focal length used (as a minimum).
If problems still exist then it likely that the cause of the problem is due to front and back focus issues and errors in focus acquisition. These errors can be confirmed by viewing the captured images within software to check if the point of focus falls in front of or behind the chosen focus point.
Front and back focus problems are errors that are associated with mirrored DSLR cameras. Unlike mirrorless models the AF sensor used to achieve focus is separate to that of the capture sensor. The AF sensor is positioned underneath the main mirror housing (see an example of a mirror box from the Canon Eos 5D Mk 3 below).
Errors in the seating of this sensor, bad or misaligned lens optics or a combination of the two can lead to minute indecencies related to focusing accuracy. These focus inaccuracies can lead to the camera and lens combination focussing in front or behind the chosen focus point(s).
Camera manufactures are aware of these tolerances and as such allow for any errors to be corrected within camera. The control to adjust these errors are known as AF Micro Adjustments (Canon) and AF Fine Tune (Nikon). Both manufacturers allow for adjustments from -20 to + 20.
A minus (-) value when applied will shift the focus error towards the camera (towards 0 thus achieving correct focus) and a plus (+) value away from the camera.
Negative values compensate for Back Focus errors in which focus has been set to behind of the chosen point
Positive values compensate for Front Focus errors in which focus has been set to in front of the chosen point.
This phenomenon does not affect mirrorless cameras as focusing is achieved directly off the camera’s capture sensor so generally there is little chance of focus errors occurring.