If you look through binoculars that are not adjusted properly, you may encounter image blackout.
Image blackout can occur when your eyes are not aligned with the exit pupil diameter or when the height of the eyecups is not properly adjusted.
When your eyes are at the eyepoint, you should get a full angle view without any blackout (see below).
There are two kinds of image blackout: peripheral blackout and so-called “kidney bean blackout.”
- peripheral blackout
This kind of blackout occurs when eyeglass wearers look through binoculars that have a short eye-relief.
This problem can be solved by getting your eyes closer to the ocular lenses. Unfortunately, your eyeglasses are in the way.
The solution is to buy binoculars with long, 15mm or more, eye-relief. 15mm eye-relief is long enough for eyeglass wearers to get a full view.
- kidney bean blackout
Kidney bean blackout gets its name from the kidney bean-shaped shadow in the field of view. You’ll often find this type of blackout when you use high eye relief binoculars with eyecups that are too short.
In either case, the blackout will disappear if your eyes are in the right position. The position where you can obtain a full angle view is called the eyepoint.
Adjusting the height of the eyecups is needed to get a full view without any blackout.
How generous the eyepoint is varies depending on each model.
With some binoculars, however, a blackout can occur even when an experienced person uses them the right way. Image blackout occurs when your eyes are even slightly off the eyepoint.
A pair like this is tiring to use because we have to hold binoculars so still to prevent a blackout.
You won’t know how generous the eyepoint is until you look through the binoculars. There is no barometer for it. In my opinion, it’s essential to know if you are comfortable with the leeway of the eyepoint before you buy a new pair.
According to Hinode Optics, various factors, such as eye-relief, the diameter of the ocular lens and exit pupil, are closely related to how much range the eyepoint has.
Generally speaking; the larger the exit pupil, the more latitude in the eyepoint. In other words, bright binoculars can easily get a full view without image blackout.
For example, large binoculars, such as 8×40 and 7×50, have larger eyepoints because the exit pupil is large.
Conversely, compact binoculars, such as 8×20, only have a 2.5mm exit pupil. So, whether compact binoculars have a wide eyepoint depends highly on their optical design.