I was initially put off from correcting an optical fault in
an old pair of Porro prism binoculars because all I had seen or read online
involved prism adjustment. Not only did this involve finding the screws, often
by peeling back and possibly damaging the casing cover but it also allowed for
the possibility of compounding, or even introducing new, errors.
Some time ago I bought a second-hand pair of Frank Nipole
binoculars from a camera shop. Although I tested them before purchasing it was
only after I had started to use them for longer periods that
I realised there
was a discrepancy between the views within each eyepiece. The presence of the
fault was made obvious when the instrument was taken away from the eyes and I
experienced a slight double vision. I realised that my eyes were rapidly and
subtly adjusting when at the eyepiece but were not so rapid in their adjustment
to unaided vision.By looking through the binoculars at a distant television
aerial and closing first one eye and then the other it became apparent that the
image from one side of the instrument was perceptibly higher than the other.
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