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photo:af_microadjustment

autofocus microadjustment

Introduction

  • PDAF autofocus systems often require calibration for a given camera and lens combination, this calibration procedure is called AF Microadjustment
    • in addition PDAF systems use an algorithm to determine the focus distance and this is generally not repeatable and gives slightly varying distances at each attempt - these are NOT fixed by AF microadjustments but are a feature of PDAF.
  • CDAF systems do NOT need micro adjustment as these systems adjust the lens focus until the the AF sensor points on the main capture sensor determine the AF position and this is extremely accurate - when it works.

  • if you use a dSLR and shoot at shallow depth of field, then you MUST do AF Microadjustment calibrations or you run the risk that ALL of your shots with that lens will be not quite right!
  • mirrorless cameras using only PDAF should also be included in this category

Why do PDAF systems need calibration?

  • two main reasons:
    • dSLR PDAF sensors are in a separate physical location to the capture sensor
      • this introduces potential variances of lens to PDAF distance vs lens to capture sensor distance, and this may even vary with temperature as metal expands and contracts
      • this is NOT a problem with mirrorless cameras with PDAF as their PDAF sensors are on the capture sensor
    • lens build variances
      • the PDAF sensor and the camera's algorithms determine what the focus distance should be and then instruct the lens AF motor to set the focus at that position
      • a small variance in build can create a repeating error with that lens and camera combination, and this may be different for each zoom position and focus position and whether or not you are using a teleconverter
      • any mechanical variances of lens mount thickness, or even wear and tear of the lens AF system such as stretching of AF wires or wear of the gears may alter these “calibrated” distances

How to perform an AF MicroAdjustment

  • this depends upon the camera model
  • the latest Nikon cameras have an automatic mechanism for performing this
  • some photographers use commercial software to assist such as:
  • others use a flat focus target they download or create as one needs to be able to accurately assess the auto focus - preferably with the focus target at the usual focus distance which the user shoots for that lens
  • set the camera up on a sturdy tripod
  • set the focus target at the desired distance and is perfectly parallel to the camera
  • take 5 shots at each calibration setting then either use commercial software, or you eye to assess which of the calibration settings gives the sharpest results - you probably should start at main large increments of calibration to get you to the ball park, and then re-test at +/-1 or +/-2 calibration points around the best point
  • On Olympus OM-D cameras (you probably only need to do this when using Four Thirds lenses):
    • Menu: Cog K (Cog A2 on the E-M1 II): AF Focus Adj: Lens Data
    • you can the manually adjust each of the AF points for the attached lens (the E-M1 II allows setting both Tele and Wide calibrations at each point when using a zoom lens)
    • if you don't want to use these settings, set AF Focus Adj to OFF.
photo/af_microadjustment.txt · Last modified: 2017/10/31 13:17 by gary1