the binos with the one open eye (now usually your right) and then instead of turning the focussing wheel, you turn the diopter adjustment wheel located in front of the focussing wheel until the opened eye is now also perfectly focused on the same object. Once done, the binoculars will be perfectly calibrated to your eyesight. Take a look with both eyes at the object you used and it should be perfectly in focus. There are also a few binoculars like the very high spec Swarovski EL binoculars pictured on the right have the diopter adjustment setting integrated into the actual focussing wheel itself. The advantage of this over the ones above is that your setting cannot accidentally be changed when turning the focusing wheel during normal use. To adjust the setting on these, first you focus the binos on something as normal, but with only one eye open (usually your left). Then you pull back on the focussing wheel which engages the gearing on the diopter and exposes the graduated scale. Now close the opposite eye to the first step and look through the binos with the open eye and then just turn the wheel as you would during normal focussing, because the diopter gearing is engaged, the wheel now usually turns with a number of click-stops and you continue until the opened eye is now also perfectly focused on the same object. Then you just click the diopter adjustment/focusing wheel back into place to lock in your setting