I recently posted a thread in the Beginners Forum on the behavior of a Vivitar 75-150 mm close-focusing lens, but did not receive a technical explanation to my question. One respondent said he had similar behavior with one of his lenses, but did not know why. I'm trying again in this SLR Lens Forum where I believe the lens experts hang out.
I recently acquired this lens, which is the two-touch type, i.e there is one ring for changing the focal length and another for changing the focusing distance. I found that when I set the focal length ring to "close focusing" I have to move the lens to about 12 inches from the subject to get a sharp image. I cannot tweak the sharpness by moving the focusing ring. Even if I move the ring from 5 ft. to infinity it has no effect on the sharpness. It seems that setting the lens to "close focusing" decouples the focusing mechanism and one just has to move the camera to the point of sharpest image. This doesn't make sense to me because moving the focusing ring greatly changes the length of the lens and the lens elements physically move, yet this does not affect the focus. How can lens elements change position without altering the focus? Please enlighten me.