Bruce and Jean had a mixed marriage. Jean was a committed birder. Bruce wasn’t. After a few bird walks with my wife and me, Bruce let on that there really is something to this birding business. He told Jean that he was ready to take the plunge, and asked her how she would feel about buying him a pair of good binoculars for his birthday. The four of us went to the store at Cape May Bird Observatory and together agreed that the Nikon LX L 8x32 would be a great choice. Bruce's excitement faded quickly. Within a few hours he was ready to return them because the image kept blacking out. Fortunately, I happened to have a few pony tail holders with me and came up with a quick fix.
Image blackout is a common problem for eyeglass wearers, especially those who don't use their binoculars often enough to instinctively center their pupils within the binocular's exit pupil. Image blackout is made worse by binoculars with wide-angle eyepieces and long eye relief. (Eye relief is the maximum distance that your eye can be from the eyepiece and still be able to see the entire field of view.) You need long eye relief to allow you to use your binoculars while wearing eyeglasses. You need a wide angle of view to make it easier to find birds. Unfortunately, desirable design features often come with a trade off.
The way to deal with image blackout is to spend enough time with your binoculars to "learn" to use them. In other words you need to instinctively center the pupil of your eye in the binocular's exit pupil. (The exit pupil is that little disk of white light that you see when you hold your binoculars up to the light.)
What is a person to do? Try this simple fix. Unless your binoculars are very old or very cheap, they have adjustable eyecups which screw, push/pull, or fold in and out. (If they don't, it is time for new binoculars.) Extend the eyecups of your binoculars and wrap an elastic pony tail holder (they look like fabric covered rubber bands) between the eyecup and body of the binocular and then retract your eyecups. (Do this for both barrels.) The pony tail holders will act as spacers to keep the eyecups from fully retracting. Now look through your binoculars. The image blackout should be gone. As you get used to your binoculars over several weeks or months remove the pony tail holders and retract the eyecups. You may find that you have developed enough skill or experience with your binoculars to no longer need the spacers.
TIP: Use pony tail holders, not rubber bands. If you use rubber bands the plasticizers will migrate out in the sunlight and gunk up your binoculars.