, ▁tradition ally ▁have ▁used ▁independent ▁foc using . ▁Central ▁foc using ▁is ▁an ▁arrangement ▁which ▁involves ▁rotation ▁of ▁a ▁central ▁foc using ▁wheel ▁to ▁adjust ▁both ▁tub es ▁together . ▁In ▁addition , ▁one ▁of ▁the ▁two ▁e y ep ie ces ▁can ▁be ▁further ▁adjust ed ▁to ▁compens ate ▁for ▁differences ▁between ▁the ▁vie wer ' s ▁eyes ▁( us ually ▁by ▁rot ating ▁the ▁e y ep ie ce ▁in ▁its ▁mount ). ▁Because ▁the ▁foc al ▁change ▁effect ed ▁by ▁the ▁adjust able ▁e y ep ie ce ▁can ▁be ▁measured ▁in ▁the ▁custom ary ▁unit ▁of ▁ref ract ive ▁power , ▁the ▁dio pt re , ▁the ▁adjust able ▁e y ep ie ce ▁itself ▁is ▁often ▁called ▁a ▁dio pt re . ▁Once ▁this ▁adjust ment ▁has ▁been ▁made ▁for ▁a ▁given ▁vie wer , ▁the ▁bin oc ular s ▁can ▁be ▁ref oc used ▁on ▁an ▁object ▁at ▁a ▁different ▁distance ▁by ▁using ▁the ▁foc using ▁wheel ▁to ▁adjust ▁both ▁tub es ▁together ▁without ▁e y ep ie ce ▁read just ment . ▁ ▁There ▁are ▁" focus - free " ▁or ▁" fixed - focus " ▁bin oc ular s ▁that ▁have ▁no ▁foc using ▁mechanism ▁other ▁than ▁the ▁e y ep ie ce ▁adjust ments ▁that ▁are ▁meant ▁to ▁be ▁set ▁for ▁the ▁user ' s ▁eyes ▁and ▁left ▁fixed . ▁These ▁are ▁considered ▁to ▁be ▁comprom ise ▁designs , ▁su ited ▁for ▁convenience , ▁but ▁not ▁well ▁su ited ▁for ▁work ▁that ▁falls