▁focus ▁on . ▁This ▁distance ▁var ies ▁from ▁about ▁ 0 . 5 ▁m ▁to ▁ 3 0 ▁m , ▁depending ▁upon ▁the ▁design ▁of ▁the ▁bin oc ular s . ▁If ▁the ▁close ▁focus ▁distance ▁is ▁short ▁with ▁respect ▁to ▁the ▁magn ification , ▁the ▁bin oc ular ▁can ▁be ▁used ▁also ▁to ▁see ▁particular s ▁not ▁visible ▁to ▁the ▁n aked ▁eye . ▁ ▁E y ep ie ces ▁ ▁Bin oc ular ▁e y ep ie ces ▁usually ▁consist ▁of ▁three ▁or ▁more ▁l ens ▁elements ▁in ▁two ▁or ▁more ▁groups . ▁ ▁The ▁l ens ▁fur th est ▁from ▁the ▁vie wer ' s ▁eye ▁is ▁called ▁the ▁field ▁l ens ▁and ▁that ▁closest ▁to ▁the ▁eye ▁the ▁eye ▁l ens . ▁ ▁The ▁most ▁common ▁configuration ▁is ▁that ▁invent ed ▁in ▁ 1 8 4 9 ▁by ▁Carl ▁K ell ner . ▁ ▁In ▁this ▁arrangement , ▁the ▁eye ▁l ens ▁is ▁a ▁pl ano - con ca ve / ▁double ▁convex ▁ach rom atic ▁double t ▁( the ▁flat ▁part ▁of ▁the ▁former ▁facing ▁the ▁eye ) ▁and ▁the ▁field ▁l ens ▁is ▁a ▁double - con vex ▁sing let . ▁ ▁A ▁revers ed ▁K ell ner ▁e y ep ie ce ▁was ▁developed ▁in ▁ 1 9 7 5 ▁and ▁in ▁it ▁the ▁field ▁l ens ▁is ▁a ▁double ▁con ca ve / ▁double ▁convex ▁ach rom atic ▁double t ▁and ▁the ▁eye ▁l ens ▁is ▁a ▁double ▁convex ▁sing let . ▁ ▁The ▁reverse