At first Canon recommend 50X. Then the manual and other documents said best at the location you are shooting. Lens Align and others said 25X. I may even have a Canon document that says 25X but I can't back it. I'll have to look in the vault. I know I have other documents (besides the manual) that say best at the location and 50X.
If you read the entire article you find this. That is what originally tuned me off to MFA. "If you think you can do better". I hope the person servicing the engine of the jet I will fly does not come up with a better method that Boing's service manual Canon - you engineered it - you tell me how to do it correctly
I, too, have 50X burned into memory as the recommended distance by Canon. Furthermore, it IS POSSIBLE to find substiation of this distance in Canon publications, such as the one at the end of this reply.
In this POTN discussion about MFA and FoCal, one of the things I also recall about camera focus was (stated by Canon, paraphrased: ) 'to within one DOF zone' or something to that effect. Still trying to find that. But... (a light bulb turns on faintly...)
- So if Tom Reichner has issues with 800mm at 250 yards (BTW, Tom, just how do you know you are at 250yards for your testing?!)
800 * 50 = 40000mm, or 1575 inches. For 800mm f/5.6, the 20/20 vision DOF is 11.5" so accuracy of AF is achieved when it is within 11.5", if my interpretation of the Canon statement (which I cannot find at the moment) is a correct one.
- Or for Larry20d and his lens at 400mm, your 30' basement is not going to 'cut it', as 50x is 66' away, so you need to buy a new home to conduct your test, or go to a high school gym that you can get into, as the basketball courts are supposed to be 74' long. And 400mm f/5.6 at 66' has a 20/20 DOF zone of 11.5", so that is the best you should expect to achieve.
- Or for Digital Paradise and his 100-400 II and 1.4 III, for 92' of indoor space (plus a bit more to stand behind your camera) you need to get into an NBA or NCAA 94' basketball court. And And 560mm f/5.6 at 92' has a 20/20 DOF zone of 11.5" as well, so that is the best you should expect to achieve, also!
One Canon 'paper' on the topic of MFA is at https://learn.usa.canon.com …ate_EOS_AF_QuickGuide.pdf
"There is no one “right” way to perform a microfocus
adjustment; however, the following method has
the benefit of being accurate and easy to perform...
1. Mount the camera and lens on a tripod. If the lens has
IS, shut it off.
2. Use Live View to manually focus on a stationary,
flat, high-contrast object that is at the center of the
viewfinder and parallel to the plane of focus. The
camera-to-subject distance should be no less than 50
times the focal length of the lens. For a 50mm lens this
would be at least 2.5 meters, or approximately 8.2 feet.
3. Focus the lens at its maximum aperture. Use Live View
magnification if necessary to assure that the image is
as sharp as possible.
4. Without touching the focusing ring or moving the tripod,
turn off Live View, and return the camera to One-Shot
AF, using only the center AF point.
5. Gently press the shutter button down halfway (or the
AF button if using back-button AF) while observing the
focusing ring or scale on the lens. It should not move.
If it does, take note of whether AF moves the plane of
focus closer (front-focus) or further away (back-focus).
If there is no shift in focus your lens is well-calibrated
and requires no adjustment.
6. To determine the correct amount of adjustment
necessary, take three sets of images at microadjustment
settings of -10 , 0 and +10; in other words,
three consecutive images at -10, three consecutive
images at 0, and three consecutive images at +10.
7. Examine the resulting images on your computer
monitor at 100% pixel magnification.
8. Take additional sets of test images at different
microadjustment settings if necessary until you can
determine which setting produces the sharpest image.
9. Register the corresponding microadjustment setting in
 Oh, I just read that Tom is using 800mm on a 1.3 crop body! In that case, DOF at 800mm f/5.6 is a zone of 9.2" depth, so you can achieve a tighter criteria of AF accuracy!