Several posts back, above,
Rich V mentioned binoculars with two hinge axes,( hopefully manufactured parallel to each other).
Changing the IPD on such , even from 56mm to 74 mm. will have a much smaller effect upon the parallelism , or lack of it, of the two optical axes, than would a 56mm to 74mm swing of a one-hinged handheld which were to suffer from lack of parallelism of its hinge and one or both optical axes.
To check the parallelism of either optical axis with its hinge, a peek- around rhomboidal beam combiner (qv via searching this forum ) can be used, at least if the line of sight is a straight shot , not inclined, toward the target. Your servant has done this on Porro II's including Nikon 20 x 120, I and II, Fuji 25 x 150 I, and Fuji 15 x 80, while prism sliding in tiny increments, sometimes via fine pitch setscrews added to minimize tiny guesswork increments, after setting the eccentrics to neutral ( or leaving the eccentrics in wherever they are locked by bad corrosion) .
Inclined binocs require additional equipment.
Swing the hinge through a large angle ( with the IPD connector disconnected), 60, 180, or even 360 degrees rotation. Observe the motion of the direct target view superimposed upon the target image seen through that side of the binocular. Ideally, there is no relative motion.
If there is motion, one can use the storied (deja vu, use search engine) mental isosceles triangle as a straightforward adjustment guide with a minimum of steps in a logical, vs. guesswork, procedure.
In practice, setting 2-hinge equipment at 65mm, often suffices, if the lines of sight are nearly
parallel, under the reasonable assumption that the hinges of any but the most sloppy manufacture are parallel.
Please note the absence here of the word "collimation", with its multiple meanings , so often confused.
It means , in the neighbor ATM forum, something entirely different than often used here. For this binocular forum, what should one write if describing adjustment of an objective lens, after passage through reflecting prisms or mirrors, such that its optical axis and the optical axis of its eyepiece are colinear, that is that they are the same line, not two parallel lines? For that, collimation is the correct word.
Edited by Gordon Rayner, 28 May 2016 - 05:02 PM.