A Question of Alignment
by Michael and Diane Porter
My binoculars make me feel like my eyes are being squeezed. After I look away I feel slightly crosseyed. If I use my binoculars for a long time I get a headache. What is going on?
You're describing some of the things that happen when binoculars are out of alignment.
What is alignment?
The binocular is a pair of telescopes mounted side by side, each supplying an image to one eye. Your eyes and brain blend the two images into one. For this magic to happen, the two telescopes have to be parallel, in alignment.
But things can happen to binoculars. If they're dropped, or knocked, or sat on, for example, the alignment between the two telescopes can be messed up. No longer will they point in exactly the same direction. Then your eyes will strain to bring the two images into alignment, causing strain and headaches.
You can't tell just by looking at a binocular whether it is in perfect alignment. That takes very expensive equipment. However, you can check for gross misalignment. Here's how to do it.
Pick out a horizontal line, such as a power line or a roof line. Look at it through your binoculars.
While you're looking through the binocular at the horizontal line, slowly pull the binoculars away from your eyes, about two or three inches.
Binoculars in alignment
The image will begin to separate into two smaller circles, like this:
Notice in the image above that the power line makes one continuous line through both circles. That means the binoculars are in alignment.
Binoculars out of alignment
If the two sides of the binocular are not in alignment, the horizontal line will step up or down between the two circles, like this:
When binoculars are out of alignment, the two images will be fighting with each other. Your brain and the muscles in your eyes will have to strain to pull the two parts into line, so that you see a single image. This effort is likely to cause a headache. Or it may just make it feel like it's not much fun to look through the binoculars.
Alignment stays even when binoculars tilted
Notice that the break in the horizontal line does not result from merely tilting the binoculars to one side. If you do tilt the binoculars, the two circles will not stay even, and that is OK. However, if the binocular is in alignment the horizontal line will still show one continuous line. Like this:
What to do about misalignment
If your binocular is out of alignment, you can't fix it yourself. You may be able to send it to the manufacturer for repair. If it is under warranty the manufacturer may repair it for a small sum or for free. (Warranties vary from manufacturer to manufacturer.)
Or replace it. If it's not under warranty, it's probably not worth paying to get it fixed, unless it's a very valuable binocular. You can purchase a new binocular that is in good alignment for less than it might cost to have a binocular realigned.
Note that the cheaper the binocular, the more easily it is likely to get knocked out of alignment. High-end binoculars have higher quality engineering, so they're better able to withstand the normal shocks of a binocular's life without going out of alignment. And if they do get out of alignment, the manufacturer is more likely to be willing to fix them.
Copyright 2009-2012 Michael and Diane Porter