big binos, porro prism rotation

Hi again. I posted a question about this to the telescope making discussion, but got no takers. Hoping to have better luck here.

Many of the bigger binoculars, especially higher end ones, seem to have fixed barrels, and allow for interocular distance adjustment by rotating their porro prisms. Lots of large military units do the same thing. Anyway, what I want to know is whether anyone knows if the rotations are always completely independent for each eye (which could lead to images not unifying?) or can be linked, say by gear teeth or other mechanism. Thanks.

Jess Tauber

Hi Jess,

My Astromeccanica/Borg 100ED binoscope had completely independent rotation of the two sides. This instrument used mirrors instead of prisms but the principle is the same. There were many times that they were not precisely symmetric about the center line and it had no effect on the merging of the images at powers <50x. Of course both eyes should see the same image orientation regardless of what angle your head is to the instrument, because both eyes move together as you rotate your head to accomodate the asymmetry.

As magnification was increased to 100x or more, I began to see the images move around a little as I rotated the two sides to change i.p.d., even when they remained symmetric. Simply changing eyepieces or the diopter adjustment if it rotates the eyepiece can cause this as well. It then becomes absolutely critical that there is an easy field adjustment for bionocular collimation, which Astromeccanica provides. At star parties I normally kept the bino at 27x so the images would merge for everyone regardless of i.p.d. or symmetry of the two sides.

Clear skies, Milt