How did you calculate the distance of the focus travel for the lens?
Ooops ! yes, Gordon, I simply use Newton's formulae ; the displacement of the image of an objet locaced at a distance D, image moving with respect to the focal point (where one finds the image of infinity), is equat to (f*f)/D or f^2/D where f is the focal length and D the finite distance from the lens to the object.
If we take one metre / about 3 feet as the reference distance D, the formula is simply
displacement in mm = (focal lenth in mm)^2 / 1000
e.g. for a focal length of 100 mm you find 1 cm or travel from "infinity" setting to "one metre" ; this is very comfortable with a classical knob, 2 cm per one full turn.
With a 55 you get (55x55)/1000 = 3 mm this is without question accessible to the same focusing knob, 3 mm is 1/6 of a turn 54 degrees of rotation.
With a 35 mm you get (35x35)/1000 = 1.2mm this starts to be difficult @20mm per knob turn,
and with a 28, you get about .8mm, this is really problematic with a conventional rack and pinion@20mm per knob turn, but easy to achieve with an helical , at least if you know where the focus is located.
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