Camera and Lens Focus Calibration

I have previously mentioned the importance of calibrating lenses and camera bodies for focus performance.  Cameras and lenses made by the major manufacturers like Canon and Nikon are very closely controlled for tolerances.  Yet, tolerances still exist that mandate checking and calibrating to make sure the auto focus system is actually focusing on the focal plane of the sensor. Usually lens and camera bodies are either front or rear focused.  A standard practice the past few years has been to use inclined plane tools that have a focus target and an inclined ruler running front to rear.  The camera is focused on the target, a shot is taken and then the ruler is analyzed to see how the depth of field centers on the focal plane. If it does not (which is usually the case), the camera body is then adjusted through MA on a Canon or AF Fine tune on Nikon.  The MA or AF setting is adjusted in increments and more shots are taken until the DOF centers on the focal plane of the target. The method takes time and is somewhat subjective.  

Recently my good shooting (Competitive rifle and camera) friend Bob Adamowicz introduced me to FoCal software from Reiken (http://www.fo-cal.co.uk) in the UK.  It is a software developed by Rich Meston of Reiken.  The program can be purchased and installed on your computer (right now only PC's) and then the camera is hooked to the PC, a target provided by Reiken is hung and properly lighted and you let the program do the work.  It runs the camera through MA (AF) settings from -20 to +20 and takes multiple shots at each setting to determine repeatability.  Then it predicts the best MA (AF) fit for that camera body/lens combination.  In addition it evaluates the relative value of the sharpness of the image. The pro version of the software provides a detailed pdf report along with a graph of all the points shot for the test.  The one shown at the top of this blog entry was my MK IV with Canon 600 f4 attached.  The quality of focus averaged about 2300 and the repeatability of quality focus was excellent throughout the curve.  The predicted MA (-5) agreed with the MA I had arrived at using the inclined rule method and the shoot tree bark at close range method :).  The key is it did it in about 3 minutes versus hours and it gave me a quantitative feel of the repeatability of the lens.  

I then tested the same lens with my 1.4 TC on it and it showed very good repeatability.  The IQ number can only be compared at same Target, aiming point, lens focal length and camera crop factor at current time. Rich is working on an algorithm so that a lens without TC can be compared to a lens with TC for relative quality of focus. 

It proved exceptionally helpful today while helping a fellow bird shooter.  He had been struggling with his 7D and 500 f4 particularly with TC mounted.  He was shooting from a Gitzo Tripod with ballhead and Wimberley Sidekick.  The test failed to perform completely twice with the 7D and 500 and when it did complete, it indicated that the prediction of MA was good, but the variability of shot to shot accuracy was high.  We then put the 7D and 500 on a Gitzo Tripod with Wimberley II Gimbal head. The test completed rapidly and yielded the same MA -but the variability of shot to shot accuracy was extremely low. 

Here is a graph of the 7D 500 f4 with Sidekick

Here is graph of 7D 500 f4 with Wimberley II

Bob Adamowicz and I have been using a long lens support system to reduce the vibration of long lens.  Initial testing that I have done with the Canon 600 shows that there is a positive shift in image quality using the support system.  We will be reporting on the Jim-Bob support system with pictures and sales details in the near future. 

I will also be shooting more tests with different supports and weighted versus non-weighted to determine a quantitative look at some of the commonly held beliefs on how to use long lenses.

Until Next time

Jim Borden