▁and ▁ λ ▁is ▁the ▁electron ▁w avel ength : ▁ ▁For ▁a ▁ 2 0 0   k V ▁mic ros cope , ▁with ▁partly ▁corrected ▁spher ical ▁aber r ations ▁(" to ▁the ▁third ▁order ") ▁and ▁a ▁Cs ▁value ▁of ▁ 1   µ m , ▁a ▁theoretical ▁cut - off ▁value ▁might ▁be ▁ 1 / q max ▁= ▁ 4 2   pm . ▁The ▁same ▁mic ros cope ▁without ▁a ▁correct or ▁would ▁have ▁Cs ▁= ▁ 0 . 5   mm ▁and ▁thus ▁a ▁ 2 0 0 - pm ▁cut - off . ▁The ▁spher ical ▁aber r ations ▁are ▁suppress ed ▁to ▁the ▁third ▁or ▁fifth ▁order ▁in ▁the ▁" ab err ation - correct ed " ▁micro scop es . ▁Their ▁resolution ▁is ▁however ▁limited ▁by ▁electron ▁source ▁geometry ▁and ▁bright ness ▁and ▁chrom atic ▁aber r ations ▁in ▁the ▁objective ▁l ens ▁system . ▁ ▁The ▁frequency ▁domain ▁representation ▁of ▁the ▁contrast ▁transfer ▁function ▁may ▁often ▁have ▁an ▁oscill atory ▁nature , ▁which ▁can ▁be ▁tun ed ▁by ▁adjust ing ▁the ▁foc al ▁value ▁of ▁the ▁objective ▁l ens . ▁This ▁oscill atory ▁nature ▁implies ▁that ▁some ▁spatial ▁frequencies ▁are ▁faith fully ▁im aged ▁by ▁the ▁mic ros cope , ▁whilst ▁others ▁are ▁suppress ed . ▁By ▁combining ▁multiple ▁images ▁with ▁different ▁spatial ▁frequencies , ▁the ▁use ▁of ▁techniques ▁such ▁as ▁foc al ▁series ▁reconst ruction ▁can ▁be ▁used ▁to ▁improve ▁the ▁resolution ▁of ▁the ▁T EM ▁in ▁a ▁limited ▁manner .