No John, on most lenses you have a bit of leeway.
The screws that hold the focus ring in place allow the helical to be moved to a somewhat different place on the turn that begins the mate. This would 'make up' for the 'incorrect, exact alignment' that you were 'supposed' to have at the theoretically exact mating location. The narrow brass ring that matches the body of the lens to the helical has narrow threads on the outside. Those narrow threads allow for ONLY ONE point in the 360 degrees to mate to the lens' body. However, the inner threads, (they are coarser), allow for about three or four points on the 360 degree turn to mate to the helical. THIS is what we are talking about when we speak of the exact point on the turn to mate: mating the helical to that narrow brass (or aluminum) ring.
Yes, It is difficult to describe in words, but, in essence, these are the two ways to get that precise maximum focus that is infinity (ie, not before or after, but exactly infinity). In other words, if you put the lens back together again and the lens does not quite focus to infinity (or slightly exceeds infinity), you can adjust those screws that hold the focus ring in place to allow the focus to achieve infinity (ie, the true 'infinity' distance from the rear element to the film plane can thus be achieved). With SLRs this is handily achieved without too much fuss because of that VF prism which guides you to the correct determination.
Of course, that determination will be accurate ONLY if the VF focus is matched, precisely, to the film plane focus. That is where the angle of the mirror become paramount in importance; slight deviation in its height can forward false information, allowing one to think his/her lens is not sharp when the lack of apparent sharpness is the result of the lens not being in REAL focus (even though the VF says that it is!!!). That mirror rests upon either a prong or a set screw which is the sole determinant as to the mirror angle. It is very important that that angle be correct in order to match the VF (apparent) focus with the film plane (actual) focus. With set screws (Zenit, Fuji ST) all you have to do is adjust it. With prongs (K1000, Minolta SR-T) you must, ever so slightly, bend it. Cardinal rule: IF ACTUAL (FILM PLANE) FOCUS IS IN FRONT OF APPARENT (VF) FOCUS, THAT MIRROR HAS TO BE LOWERED SLIGHTLY. IF ACTUAL (FILM PLANE) FOCUS IS IN BACK OF APPARENT (VF) FOCUS, THAT MIRROR HAS TO BE RAISED SLIGHTLY.
Honestly, John, I think that Nikon alone used the 'white' grease, and appropriately sparingly at that. The others seemed to use either Vaseline or axle grease (!) and used too much so that, with time, those lenses attained a stiff focus when the grease dried out (horribly stiff in cold weather!!!), leaving much solid detritus behind to clog those narrow threads!. Less would have been more but 'dampening' and making the focus turn smoother (only when new!!!) was too trendy and trumped pragmatism. Time revealed this manufacturing error. - David Lyga
Last edited by David Lyga; 10-20-2013 at 04:28 PM. Click to view previous post history.