Kevin Purcell
Well-known member
My impression of the non standard focus direction is that the elements used in the focus operation still work in the same direction. This doesn't change. What changes is simply the direction of the cut of the gears (LH thread vs RH thread), so some need to move CW and others CCW. I have always assumed that a simple change in gears would then change focus wheel direction, but not the focus lens direction.
Well that depends on which direction you need to get the lens to move. But it would be ore likely a change of sense of a thread on a screw or a worm gear.
But I have to admit to being somewhat confused by mayoayo. It sounds like the suggestion (implication) is that there is grease inside the lens mechanism. Now I thought that the focusing was accomplished by a rod which is outside the lens assembly and that thus any lubricating grease (which would be on the rod and the gears associated with moving the rod) is therefore also outside the lens assembly and should not be able to be viewed at all. I can't imagine a decent binocular design that would let grease into the lens and prism housing.
This seems to be quite common. There is grease (probably a synthetic low vapor pressure grease) that lubes the focuser lens mounting that slides in the machined surface of the barrel.
That grease isn't going to go anywhere else either. It's sticky and there isn't enough to run elsewhere in the system. Plus the bin is sealed so no dust should get in there to gunk it up.
I can think of other ways of doing it but they all seem to be very high tolerance mechanisms (i.e. expensive!).
Other bins put the focuser assembly in a different location (close to the SP prism) so it sort of hangs off the focuser mechanism in mid-air (well, mid-nitrogen or mid-argon) and doesn't contact the sides of the bin.
I suspect there are some other useful optical effect that leads to this sort of design like reducing aberrations perhaps with a minimum number of lenses e.g. the two objective and focuser could be different glass types or could be an sort-of-achromat followed by another lens so you get an APO effect focusing light at three wavelengths. So you you deal with a CA problem and make the bin able to focus too when you optimize the whole objective system. After all adding a larger focuser lens increases weight and cost of good so you need a good reason to put it in. Perhaps mechanical simplicity is part of the driving force too.
Tero: the lenses won't touch ... that focuser lens mount usually projects in front of the frontmost part of the focuser lens so it can't touch the objective even when forced to the end of it's travel.
Last edited: