binocular's diopter adjustment lets you alter the focus of one
eyepiece in order to compensate for any difference between your
left and right eyes. With the diopter adjustment properly set,
both eyes will comfortably focus at the same distance.
classical diopter adjustment, still used by most Porro prism
and some roof prism binoculars, allows you to turn one eyepiece
to adjust its focus. On many binoculars, however, the slightest
bump undoes the adjustment.
however, top roof prism binoculars lock the diopter setting
so that it doesn't accidentally get changed. A ring must be
pulled or a knob pushed before the diopter adjustment will move.
This is a small but valuable improvement.
binoculars that employ such an advance diopter adjustment locking
mechanism include the following:
Nikon Premier LX
Pentax DCF SP
Zeiss Victory FL T*