▁is ▁too ▁large , ▁the ▁beam ▁quality ▁may ▁not ▁be ▁improved ▁as ▁much ▁as ▁desired . ▁ ▁The ▁size ▁of ▁a pert ure ▁that ▁can ▁be ▁used ▁also ▁depends ▁on ▁the ▁size ▁and ▁quality ▁of ▁the ▁opt ics . ▁To ▁use ▁a ▁very ▁small ▁pin hole , ▁one ▁must ▁use ▁a ▁foc using ▁l ens ▁with ▁a ▁low ▁f - number , ▁and ▁ide ally ▁the ▁l ens ▁should ▁not ▁add ▁significant ▁aber r ations ▁to ▁the ▁beam . ▁The ▁design ▁of ▁such ▁a ▁l ens ▁becomes ▁increasing ly ▁more ▁difficult ▁as ▁the ▁f - number ▁decre ases . ▁ ▁In ▁practice , ▁the ▁most ▁commonly ▁used ▁configuration ▁is ▁to ▁use ▁a ▁mic ros cope ▁objective ▁l ens ▁for ▁foc using ▁the ▁beam , ▁and ▁an ▁a pert ure ▁made ▁by ▁p unch ing ▁a ▁small , ▁precise , ▁hole ▁in ▁a ▁piece ▁of ▁thick ▁metal ▁fo il . ▁Such ▁assemb lies ▁are ▁available ▁commer cially . ▁ ▁S pher ical ▁waves ▁By ▁om itting ▁the ▁second ▁l ens ▁that ▁re forms ▁the ▁col lim ated ▁beam , ▁the ▁filter ▁a pert ure ▁closely ▁approxim ates ▁an ▁inten se ▁point ▁source , ▁which ▁produces ▁light ▁that ▁approxim ates ▁a ▁spher ical ▁wave front . ▁A ▁smaller ▁a pert ure ▁implements ▁a ▁closer ▁approximation ▁of ▁a ▁point ▁source , ▁which ▁in ▁turn ▁produces ▁a ▁more ▁nearly ▁spher ical ▁wave front . ▁ ▁See ▁also ▁Kö h ler ▁ill um ination , ▁which ▁applies ▁spatial ▁filtering ▁to ▁in co her ent ▁light ▁for ▁micro sc