Anthony wrote:
I use binoculars frequently for work (wildlife surveys) and recreation (bird watching since age 15). This demands that I be able to resolve fine detail to accurately resolve multiple animals in the field of view, or to resolve fine details on an individual animal so that I can identify it to species, age, and gender.  Since I began using good pairs of roof-prism binoculars in my late 20s, I noticed that I frequently have to adjust the diopter to keep both eyes in focus even though the diopter adjustment on the binocular has not been moved. That is, if I set the diopter at zero (because my eyeglasses correct my eyesight), sometimes only one eye
will be in focus when I put the binoculars up to my eyes and sometimes both eyes will be in focus. When only one eye is in focus, I adjust the diopter (typically to beyond -2)), but after lowering the binoculars for a few minutes, then raising them to my eyes, about half the time one eye will be out of focus.  What could be the reason for this?  This is a problem whether I use my $350 Minolta binoculars or my $1,300 Zeiss binoculars.  The collimation has been checked on both binoculars and both are 7X magnification).  The problem is less noticeable with porro-prism binoculars (also 7X power), which have a greater depth of field than roof-prism binoculars.

I have been wearing eyeglasses for strong myopia and slight astigmatism since age 7.  I am now age 48.  There is a small difference in prescription between the two eyes, however, over the years my optometrists have typically given me prescriptions so that the lens for my weaker eye gives a sharp image at reading distance but not at infinity, whereas the lens for my stronger eye gives a sharp image at infinity, but not close up.  The adjustable diopter on binoculars should take care of that slight difference but the binoculars cannot be
blamed for the difference between the two eyes frequently varying from -0.25 to -3.0.  The problem with frequently needing to adjust the diopter on binoculars seems to have been present before I developed
noticeable presbyopia and at least 20 years before I began using multifocal lenses.

ANSWER

Anthony,

At age 40 you still have “some” focusing ability, and you are using this as you adjust the binoculars to clear the image. Then when you go back to use the binocular again, your focusing
will not instantly adjust.   The difference in
your two lenses in your glasses likely adds to this and I might suggest a special pair of glasses that you wear in the field that has the two eyes balanced for distance vision.  The other culprit may be the pupil distance - the separation of the lenses of the binocular.  Perhaps your Optometrist could let you know what your PD ( the distance between your
eyes) is, so that you can set that correctly on your binoculars.

I know it is frustrating when you need to quickly use your binoculars, but setting them before you leave home and using balanced glasses may help greatly.  If it is any consolation, I often have the same problem, but find it works OK when I adjust and lock them into focus for me.

Dr. Joan Hansen, Optometrist for CAO

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