Diopter range is an adjustment on optical products which can correct the prescription on each of your eyes. That way you can see a sharp image without wearing glasses. The diopter ring is normally located on the eyepiece and by turning it your image appears sharper.
Every optical product has different diopter range, from positive (+) to negative (-).
The diopter ring is present in central focusing system on each of the barrels near the eyepiece, where you can correct the difference in the prescription of the left and right eye individually. Once you have set the right value, you can focus the image with using just the central focusing ring. If you’re wearing glasses, the diopter value should be set to 0, because the differences in your eyes are already corrected in your glasses.
To see a sharp image without wearing glasses you can easily set the diopter by looking with bare eye, turning the ring and adjusting sharpness. So when looking with both eyes your image should appear sharp. If you have astigmatism the diopter adjustment cannot correct it – you’ll still need your glasses and diopter set to 0 to see sharp images.