Finally got around to getting a Bahtinov mask for the 300mm lens.
Tested it between the clouds. It is far more sensitive than checking test exposures at high magnification.
The 300mm lens is focussed by a stepper motor. One thousand steps is a movement of 0.12 mm of the focus ring.
Below is a movie going from focus in one thousand step increments to +6000 steps and back to -6000 steps.
Animated gif 5MB
http://d1355990.i49.quadrahosting.co...bahtinov01.gif
Enjoy the movie.
Bert