To properly set back focus place focus chart at 2 times the distance of minimum focus of the lens, preferably a hard mark on the
lens so that you are not guessing if that mark is really 5', but there is a mark on the lens. Open the lens wide open and put it on the
widest angle. On the eyepiece turn the peaking knob up all the way and turn down the brightness. Now loosen the back focus knob
and turn it until you see very crisp sparkles on the chart. When you have the desired crispest image hold the back focus ring while
you tighten it down. Then zoom in and out to see if it holds focus in the full range of the lens. It's very important to note that back
focus can change if there is an extreme change in the temperature.