ED(Extra-low Dispersion) glass is built in APEXEL 12x50 monocular. ED glass has this unique characteristic and when combined with other glasses minimizes the effects of the secondary spectrum. Comparing to achromatic lenses, and ED glass reduces chromatic aberration to a remarkable degree. So if you would like to observe in low light conditions, such as at dawn or dusk and it is better to use an ED glass Binocular.
BAK4 has a high refractive index and lower critical angle than other materials, which means it transmits light better with less light being lost due to internal reflectionsuch as from internal bubbles trapped during the manufacturing process.
The diopter adjustment is a control knob on your binocular. It is designed to let you compensate for differences between your own two eyes. Once you set the diopter, then the two barrels should stay in proper relation. From then on you can focus just by turning the central focusing knob.