▁K ell ner ▁provides ▁ 5 0 % ▁more ▁eye ▁relief ▁and ▁works ▁better ▁with ▁small ▁foc al ▁r ati os ▁as ▁well ▁as ▁having ▁a ▁slightly ▁wider ▁field . ▁ ▁W ide ▁field ▁bin oc ular s ▁typically ▁util ize ▁some ▁kind ▁of ▁Er f le ▁configuration , ▁pat ented ▁in ▁ 1 9 2 1 . ▁ ▁These ▁have ▁five ▁or ▁six ▁elements ▁in ▁three ▁groups . ▁ ▁The ▁groups ▁may ▁be ▁two ▁ach rom atic ▁double ts ▁with ▁a ▁double ▁convex ▁sing let ▁between ▁them ▁or ▁may ▁all ▁be ▁ach rom atic ▁double ts . ▁ ▁These ▁e y ep ie ces ▁tend ▁not ▁to ▁perform ▁as ▁well ▁as ▁K ell ner ▁e y ep ie ces ▁at ▁high ▁power ▁because ▁they ▁suffer ▁from ▁ast ig mat ism ▁and ▁g host ▁images . ▁ ▁However ▁they ▁have ▁large ▁eye ▁l enses , ▁excellent ▁eye ▁relief , ▁and ▁are ▁comfortable ▁to ▁use ▁at ▁lower ▁powers . ▁ ▁Mechan ical ▁design ▁ ▁F ocus ▁and ▁adjust ment ▁▁ ▁Bin oc ular s ▁have ▁a ▁foc using ▁arrangement ▁which ▁changes ▁the ▁distance ▁between ▁oc ular ▁and ▁objective ▁l enses . ▁Norm ally ▁there ▁are ▁two ▁different ▁arrang ements ▁used ▁to ▁provide ▁focus , ▁" in dependent ▁focus " ▁and ▁" central ▁foc using ": ▁ ▁Independent ▁focus ▁is ▁an ▁arrangement ▁where ▁the ▁two ▁teles cop es ▁are ▁focused ▁independently ▁by ▁adjust ing ▁each ▁e y ep ie ce . ▁Bin oc ular s ▁designed ▁for ▁heavy ▁field ▁use , ▁such ▁as ▁military ▁applications