▁there ▁are ▁also ▁objective ▁l enses ▁above ▁the ▁sample ▁to ▁make ▁the ▁incident ▁electron ▁beam ▁conver gent ). ▁The ▁project or ▁l enses ▁are ▁used ▁to ▁expand ▁the ▁beam ▁onto ▁the ▁ph osph or ▁screen ▁or ▁other ▁imag ing ▁device , ▁such ▁as ▁film . ▁The ▁magn ification ▁of ▁the ▁T EM ▁is ▁due ▁to ▁the ▁ratio ▁of ▁the ▁distances ▁between ▁the ▁spec imen ▁and ▁the ▁objective ▁l ens ' ▁image ▁plane . ▁Add itional ▁st ig mat ors ▁allow ▁for ▁the ▁correction ▁of ▁asym met rical ▁beam ▁dist ort ions , ▁known ▁as ▁ast ig mat ism . ▁It ▁is ▁noted ▁that ▁T EM ▁optical ▁configurations ▁differ ▁significantly ▁with ▁implementation , ▁with ▁manufact ur ers ▁using ▁custom ▁l ens ▁configurations , ▁such ▁as ▁in ▁spher ical ▁aber r ation ▁corrected ▁instruments , ▁or ▁T EM s ▁using ▁energy ▁filtering ▁to ▁correct ▁electron ▁chrom atic ▁aber r ation . ▁ ▁Re ci pro city ▁ ▁The ▁optical ▁reci pro city ▁theorem , ▁or ▁principle ▁of ▁Hel m hol tz ▁reci pro city , ▁generally ▁holds ▁true ▁for ▁el ast ically ▁scattered ▁electrons ▁in ▁an ▁absor bing ▁medium , ▁as ▁is ▁often ▁the ▁case ▁under ▁standard ▁T EM ▁operating ▁conditions . ▁The ▁theorem ▁states ▁that ▁the ▁wave ▁amplitude ▁at ▁some ▁point ▁B ▁as ▁a ▁result ▁of ▁electron ▁point ▁source ▁A ▁would ▁be ▁the ▁same ▁as ▁the ▁amplitude ▁at ▁A ▁due ▁to ▁an ▁equivalent ▁point ▁source ▁placed ▁at ▁B . ▁Sim ply ▁stated , ▁the ▁wave ▁function ▁for ▁electrons ▁focused ▁through ▁any ▁series ▁of ▁optical