, ▁is ▁after ▁the ▁sample . ▁It ▁can ▁be ▁adjust ed ▁to ▁produce ▁a ▁focused ▁diff raction ▁pattern ▁or ▁image ▁of ▁the ▁sample ▁with ▁a ▁magn ification ▁varying ▁over ▁a ▁huge ▁range . ▁Many ▁single ▁micro scop es ▁can ▁cover ▁the ▁magn ification ▁range ▁from ▁roughly ▁ 1 0 0 X ▁to ▁more ▁than ▁ 1 , 0 0 0 , 0 0 0 X . ▁ ▁Equ ally ▁important ▁to ▁the ▁l enses ▁are ▁the ▁a pert ures . ▁These ▁are ▁circular ▁holes ▁in ▁thin ▁stri ps ▁of ▁heavy ▁metal , ▁placed ▁at ▁well - ch osen ▁points ▁in ▁the ▁column ▁of ▁l enses . ▁Some ▁are ▁fixed ▁in ▁size ▁and ▁position ▁and ▁play ▁important ▁roles ▁in ▁limit ing ▁x - ray ▁generation ▁and ▁impro ving ▁the ▁vac u um ▁performance . ▁They ▁also ▁prevent ▁electrons ▁from ▁passing ▁through ▁the ▁outer most ▁parts ▁of ▁the ▁magnetic ▁l enses ▁which , ▁due ▁to ▁large ▁l ens ▁aber r ations , ▁focus ▁the ▁electron ▁be ams ▁extremely ▁poor ly . ▁O thers ▁can ▁be ▁freely ▁switched ▁among ▁several ▁different ▁sizes ▁and ▁have ▁their ▁positions ▁adjust ed . ▁These ▁" variable ▁a pert ures " ▁are ▁used ▁to ▁determine ▁the ▁beam ▁current ▁reaching ▁the ▁sample ▁and ▁also ▁to ▁improve ▁the ▁ability ▁to ▁focus ▁the ▁beam . ▁Variable ▁a pert ures ▁after ▁the ▁sample ▁position ▁further ▁allow ▁the ▁user ▁to ▁select ▁the ▁range ▁of ▁spatial ▁positions ▁or ▁electron ▁scatter ing ▁angles ▁to ▁be ▁used ▁in ▁the ▁formation ▁of ▁an ▁image ▁or ▁a ▁diff raction ▁pattern .