Lens AF Calibration explained

What follows here is taken from Brani Srnec’s educational segment on lens autofocus as presented at the March, 2016 meeting.


Contents

1.  Why to Calibrate
2.  How it works
3.  Calibration naming conventions
4.  Calibration tools
5.  Getting ready – what do we need?
6.  Camera settings
7.  Calibration steps
8.  Calibration tolerance values
9.  Prime vs Zoom lens calibration

1. Why to Calibrate

  • There is some tolerance in AF during manufacturing process
  • To get the best possible sharpness from your lens
  • To get familiar with your lens sharpness, focusing speed, accuracy, bokeh and other aspects
  • Calibration is a complex process
  • Slight autofocus issues will be more noticeable when using high resolution cameras and/or lenses with very shallow DOF (f1.2-f2.8)
  • Useful for portrait, event, wedding photographers, not so much for landscape, architecture…
Calibration 1
Example of Front Focus on a 50mm f1.8 lens. Notice that the sharp area is in front of the 00 target point.
Calibration 2
Example of adjusted focus with the same 50mm f1.8 lens after adjusting the lens calibration by +9 in the camera settings.
Calibration 3
Example of a front-focusing 50mm f1.8 lens. Auto-focus thinks it is in focus, but the image captured by the sensor is not in focus.
Calibration 4
Example of adjusted focus with the same 50mm f1.8 after adjusting the lens calibration by +9 in the camera settings.

2. How it Works

Good news:

  • The process is safe and any changes are reversible
  • No physical adjustments = no damage to the camera or lens

Bad news:

  • AF adjustment is not available in every camera
  • Calibration requires a calibration tool, time and patience
  • Calibration is lens-camera specific. Only one value can be stored for a specific lens/camera combination
  • If you use more cameras, you need to calibrate the same lens for each camera
  • You can save up values for different lenses (even values for the same lens combined with a teleconverter)

3. Calibration naming conventions

  • Nikon – AF Fine Tune
  • Canon – AF Micro Adjustment
  • Sony – AF Micro Adjustment
  • Pentax – AF Adjustment
  • Olympus – AF Focus Adjust

![AF Fine Tune screen on Nikon](AF-Fine-Tune-screen-on-Nikon_thumb.png AF Fine Tune screen on Nikon)

Calibration 5
AF Fine Tune screen on Nikon cameras
Calibration 6
AF Micro Adjustment screen on Canon cameras

4. Calibration tools

http://regex.info/blog/photo-tech/focus-chart/

Focus Calibration Tool – Free Download


Calibration 8Focus Calibration Tool – ~$2.56 from eBay


Calibration 9

Focus Calibration Tool – Focus Pyramid – US$24.95 from Amazon.com (currently unavailable on amazon.ca)


Calibration 10Focus Calibration Tool – LensAlign MkII – ~$169.14 from Amazon.ca


Calibration 11

Focus Calibration Tool – Datacolor Spyder LensCal SLC100 – $82.79 from Amazon.ca


Calibration 12

Focus Calibration Tool – automated calibration software – US$134.88 from BH Photo Video


Calibration 13

Focus Calibration Tool – Sigma USB Dock – for use with specific Sigma lenses – US$59.00 from BH Photo Video

5. Getting Ready

Yo will need:

  • Calibration tool
  • Tripod
  • Plenty of natural or artificial light
  • Tethered shooting helps to review results much faster than loading test shot to the computer and more accurate then reviewing in the camera

6. Camera Settings

  • Manual mode
  • Manual WB
  • ISO between ISO100-400
  • Shutter speed at least 1/250
  • Aperture wide open (smallest f stop)
  • Centre focusing point and AF On
  • IS/VR on the lens Off
  • Viewfinder grid On helps to level camera
  • Timer On/Exposure Delay Mode On/Cable shutter release/Mirror Up On

7. Calibration Steps

  • Place calibration tool on a flat well lit surface
  • Put camera on a tripod in a distance you normally use the lens
  • Some sources recommend about 25 times the focal length of the lens (e.g. 85mmx25=2.1m) [Source]
  • Level camera with the centre of the focusing point
Calibration 14
Position of camera relative to the focus calibration tool. The distance to the tool will vary with which lens you are calibrating.
  • Take a test image and evaluate focus accuracy on the monitor screen at 100% view
  • When the lens Front focuses, dial “+” number in the camera AF Adjustment menu
  • When the lens Back focuses, dial “–” number in the camera menu

↓ “-” back focusing

↑ “+” front focusing

  • Take another image and repeat the first or second step until the sharpest part of the image is the one you focus on
  • Don’t forget Refocusing between shots (I used my hand to put in front of the lens to force the focusing system to refocus and then I focused back on the target)
  • Take notes
  • When you think you are done, take plenty of test images to confirm your AF adjustment decision is correct
  • Take your gear for a real test outdoors. Try different distances and see if you are getting good and consistent results.
  • Periodical recalibration is recommended (once a year should be fine)

8. Calibration Tolerance Values

  • The maximum AF adjustment is +-20 increments
  • If you find that every lens you test is back/front focusing by about the same amount, it is possible that there is actually problem with the camera AF system instead of the lens AF
  • If you find that a lens needs adjustment more than 20, the lens probably needs a physical adjustment done by professional service

9. Prime vs Zoom lens calibration

  • Calibrating Prime lens is relatively easy, because you calibrate AF for only one focal length.
  • Calibrating Zoom lens can be a challenge, because you can experience different results at different focal lengths

Example:
My 70-200mm lens front focused
At 70mm it needed +1 adjustment
At 200mm it needed +7 adjustment

Problem:
you can save only one adjustment value in the camera for a specific lens. This adjustment is then applied for all focal lengths. However, there are some cameras that can save two values for one lens.

Solution:
Using average of the AF adjustment values. In my example +4. Using AF adjustment value of the focal length you use the lens most often. In my case I use the lens the most often at 200mm, so I would save +7.

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