Most people think they know how to properly focus binoculars. After all, it's just a matter of turning the focus wheel while using the binoculars- right?
In fact, this is only the case if you have 20/20 vision- which only accounts for 35% of people. It's much more common for people to have one eye weaker than the other. If this is the case for you, then having both barrels at the same focus is going to mean a slightly blurry image through one of them.
Thankfully, almost all good-quality binoculars come with something called an adjustable dioptre ring. This allows you to adjust the focus one one barrel (usually the right one) separately from the main focus wheel. This is called "calibrating" the binoculars, and it's nice and simple to do. It also only needs to be done once, so long as you don't knock the calibration off and you're not sharing the binoculars with someone else.
So how do I calibrate my binoculars?
For the sake of this post, we'll be assuming that the dioptre ring is on the right-hand barrel of your binoculars. If it's on the left, not to worry- just flip what you're doing to the other way round!
Start by looking through the left eyepiece of your binoculars, with your right eye closed. Point the binoculars at an object about 10 metres away, and adjust the focus wheel until the object is fully in focus.
Then, still looking at the same object, shut your left eye and open your right eye. If you have 20/20 vision, then the object will still be in full focus. If not, though, then you'll notice it's now slightly out of focus. This is where the dioptre ring comes into play. If your vision is better in your left eye, then you'll need to twist it towards the plus symbol. Conversely, if your vision is better in your right eye, then you'll need to twist it to the minus symbol.
Once you've calibrated the right barrel, you should find that when you look through both barrels at once, the image is perfectly clear. You're now free to adjust the binocular focus to look at objects at any distance, and the binoculars will remain calibrated to your eyes.
Some binoculars have a lockable dioptre ring so that you don't knock the calibration off. However, this tends to only be found on higher-end models- most won't have this feature.
Looking for a new pair of binoculars? Check out our Top 10 recommendations here >