Question from Accurate Instrument Servicing Pty Ltd

“Would it be correct to say it’s basically not possible to get 100% AF focus—only very close—because of mechanical or possibly optical tolerances?”

Answer

Yes, that's essentially correct. Most lenses are not manufactured to tolerances that can always achieve perfect Auto Focus. However, a good lens will consistently front or back focus by a predictable amount.

To correct this, lenses include a data file called Focus Compensation, which tells the camera body how much to adjust to achieve optimal focus. This is not an autofocus (AF) adjustment on the camera—it is lens-specific data used by the camera’s AF system as part of its focusing calculation.

Lens Focus Compensation – Quick Overview:
Focus Compensation is designed to make a lens provide the best possible AF performance across all camera bodies (assuming the camera body AF system is properly calibrated).

• It is not an AF adjustment.
• It is used by the camera as part of its autofocus calculations.
• Calibration is for the central AF frame in the viewfinder.
• Different calibration values exist for:
  • Different AF sensor types within the central frame
  • Different zoom settings
  • Different EOS AF system designs

For Canon Lens Professional Software Users

More details can be found in the Software’s ‘Help File’
Help → Using the Software → Lens Operations Menu → Focus Compensation – Adjustment Overview