In the FoCal manual, it says Canon recommends 50x focal length for testing distance. I did a DoF calculation. For 200mm, you would place the camera 10m away from the target if following Canon's recommendation. At this distance and f2.8, the DoF is 0.26m with 0.13m for both front and back. So the tolerance of in-focus is pretty loose. If you place the camera 25x focal length, the DoF is only 0.06m which is much shallower. I don't know how sensitive FoCal is in detecting the sharpness difference, but I would definitely place the camera closer in hope to improve the testing accuracy. Plus 50x focal length is too far a shooting distance in practice for non- super tele-lens. 25xfocal length appears to be a good distance to use. Any thoughts? Thank you!
AFMA settings will vary with distance. The 50X distance is not going to be optimal for those who like to get close.
Here is a screen shot of one of the tests I ran. Its pretty obvious as to where the best AFMA lies, so the main thing is to adjust it at the distance you use.
If you shoot your 200mm lens at 2 meters, then adjust it there. All lenses tend to front focus at close distances. The 50X distance applies where your typical image is not a close focus image.
I hope that explains it a bit.
With focal, you are going to get a curve like this, with the peak being obvious. Being off by a couple of AFMA units usually makes very little distance.
HOWEVER, the curve may shift left or right depending on the distance, and near mfd, it can be a big difference.