al ▁length , ▁fixed ▁foc al ▁length , ▁or ▁F FL ▁to ▁avoid ▁ambigu ity . ▁ ▁As ▁alternative ▁to ▁zoom ▁l ens ▁ ▁The ▁term ▁prime ▁has ▁come ▁to ▁mean ▁the ▁opposite ▁of ▁zoom — a ▁fixed - f ocal - length , ▁or ▁un if ocal ▁l ens . ▁ ▁While ▁a ▁prime ▁l ens ▁of ▁a ▁given ▁foc al ▁length ▁is ▁less ▁vers atile ▁than ▁a ▁zoom ▁l ens , ▁it ▁is ▁often ▁of ▁superior ▁optical ▁quality , ▁wider ▁maximum ▁a pert ure , ▁l ighter ▁weight , ▁and ▁smaller ▁size . ▁These ▁advantages ▁stem ▁from ▁having ▁fewer ▁moving ▁parts , ▁optical ▁elements ▁optimized ▁for ▁one ▁particular ▁foc al ▁length , ▁and ▁a ▁less ▁complicated ▁l ens ▁formula ▁that ▁creates ▁fewer ▁optical ▁aber r ation ▁issues . ▁Lar ger ▁maximum ▁a pert ure ▁( small er ▁f - number ) ▁facil it ates ▁phot ography ▁in ▁lower ▁light , ▁and ▁a ▁shall ower ▁depth ▁of ▁field . ▁ ▁A ▁normal ▁l ens ▁or ▁" normal ▁prime " ▁is ▁a ▁l ens ▁with ▁a ▁foc al ▁length ▁about ▁equal ▁to ▁the ▁diagonal ▁size ▁of ▁the ▁film ▁or ▁sensor ▁format , ▁or ▁that ▁reprodu ces ▁perspective ▁that ▁generally ▁looks ▁" natural " ▁to ▁a ▁human ▁observer ▁under ▁normal ▁view ing ▁conditions . ▁ ▁Trad itional ▁meaning ▁as ▁primary ▁l ens ▁▁ ▁An ▁alternative ▁and ▁older ▁meaning ▁of ▁prime ▁l ens ▁is ▁the ▁main ▁l ens ▁in ▁a ▁combination ▁l ens ▁system . ▁ ▁When ▁the ▁camera ▁l ens ▁is ▁used ▁with ▁some