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9.0& THE&CHALLENGE&OF&MANAGING&LENS&ABERRATIONS&
Optical! aberrations! are! an! inescapable! reality! of! each! and! every! element! within! all! lenses.! ! The! well-
known!Seidel!monochromatic!aberrations are!image!defects!associated!with!the!fundamental!behavior!
of!light!rays—of!all!w avelengths!—!passing!through!a!lens!element.!!These!m onochromatic!aberrations!
include! sp herical$ aberration,$ coma,$ astigmatism ,$ curvature$ of$ field$ and$ geometric$ distortion.!!!
Collectively,!they!are!often!termed!the!“defocusing!distortions”!because!they!cumulatively!conspire!to!
impair!image!sharpn es s!( es p ec ia lly!a s!a !le n s!iris !se t tin g!a p p ro a ches!maximum!aperture).
!
In!addition!t o !monoch r o m a t ic!aberrations,! the re!are!additional!aberrations!associated!with!colored!light!
that! are! wavelength-dependent.! !!Different! w avelengths! of! light! enc o u n te r! a! diffe ren t ! index! of!
refraction!w ithin !a !g iven !o p tical!material,!referred!to!as!dispersion.!!!These!are!technically!described! by!
two!aberra tions :!(a)!!Longitudinal$chromatic$aberration$(different!focus!plane!for!c on stitu en t!co lors)! th at!
produce! an! unwanted! colored! flare;! ! and! (b)! Lateral$ chromatic$ aberration$ (because! magnification! of!
differen t! wavelen g th ! rays! vary! they! cau s e ! an! associated! variation! in! the! lateral! m agnification! that!
producing!an!effective!color!misregistration!in!th e !reco rd ed !video !or!still!im age).!!
!
Light! rays! passing! thro ugh ! th e! ce nte r! of! a! m u lti-elem ent! lens! will! exhibit! a! modest! degree! of! these!
aberrations.! !How ever,!as!light! rays!from!the! extremities! of!a! framed! scene!enter! the! lens! at!increasing!
angles! and! are! ultimately! focused! on! the! extremities! of! the! CMOS! image! sensor! image! plane,! the!
aberrations!can!all!rapidly!increase.!!The!m ore!acute!the!an gle!of!those!rays!being!projected!on !to!the!
sensor! —! the! greater! the! level! of! aberrations.! !! For! a! large! image! se nso r! like! Full! F ram e! this! pose s! a!
particular!challenge!to!the!lens!—!especially!when!the!corners!of!the!image!contain!high!detail.!!!!
!
!!!!!!!!!!!!!!!!!!!! !
Figure&9!!!!!!Concept$of$a$hypothetical$len s$im a g in g$a$h igh ly $de ta ile d $scen e $—$and$the$importance$ of$the$
back$focus$distance$and$rear$lens$diameter$on$the$angle$of$a$ray$bundle$projecting$onto$the$corners$of$
the$image$sen so r$
10!
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