Owner's Manual

Table Of Contents
Two!key!dimensions! play!a! significant!role!in! the!quality!of!the!final!image!p ro jecte d! on!to! the!image!
sensor!!they! are!the!back! focus!distance!and!the!diameter!of!the!final!lens!element.!!!!Figure!9! is!a!
simplistic!illustration!of! the ir! cum ulative! effect! on ! the ! ang le! of! th e! ray!bundle! landing!on!th e! extrem ities!
of! the!ima ge !senso r.!!The!colored!circles!iden tify !ray!bundle s!affect in g!ima ge !perfo rm a n c e!in!the!image!
central!zone!(red)!and!in!the!image!extremities!(blue).!
!
As!the!refractive!index,!or!“bending”!power!of!the!lens!increases,!various!aberrations!tend!to!increase !
easily.!
!
If! the! re a r! lens! is! larger,! ray! bund les! can! be! projected! more! gently! to! th e! larger! image! senso r! and!
suppression!o f!th e!occurrence!of!various!aberrations!can !be!made.!On!the!other!hand,!the!smaller!th e!
rear!len s! is,!it! is!neces sa ry ! that!th e!refra ction ! of!th e! ray!have! a!more ! acute!angle!to! the! larger!image!
sensor!by!stronger!refractive!power!of!the!lens,!which!can !le a d !t o !more!va rio u s !a b e r ra ti o n s.!
!
The! criticality! of! the! back! focus! distance! is! simplistically! illustrated! in! Figure ! 10.! ! ! The ! top! image!
simulates! a!lens! having!a! long!ba ck! focus!d istance! —! such! as!exists! in!the! current!Canon! EOS!EF! lens!
system.!!!M any!design!strategies!are!m obilized! for!the!m any ! different!types!of!zoom! an d!prime! lenses! in!
this! fam ily! of! lenses! to! cou nte ract! the! effect! of! aberrations.! ! One! key! strategy ! is! to! use! a! lar ge ! lens!
element!in! the!front! of!the! lens!system!to! more!independently! and! gently!manage!the! degree!of!the!
angle!of!the!emerging!light!ray!bun dle!p rojected!on!to!the!im age!sensor.!!This!generally!will!extend!the!
length!o f!th e !o ve ra ll!o p tic al!s ys te m.!!!
If,!ho wever,! th e !back!focus !distan ce !could !be! s h o rt e n ed ,!this! th en ! o pe ns ! u p ! sp ac e!to!m o ve ! th e!final!len s!
element!closer!to!the!image!sensor!—!and!if $this$elemen t$is$made$large!then!an!equivalent!control!fo r!
aberrations! of!the!ray!bundle!projected!on!to!th e!corner!of!the!im ag e!sensor!can!be!made!!as!shown!
in!the!lower!image !in !F igu re !10.!
! ! ! ! !
Figure&10! Showing$two$alternatives$in$lens$design$to$indepe n d en t ly$and $gent ly$control$aberrations$
of$the$ray$bundle$projected$on$to$the$corner$of$the$image$sensor&
11!