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7/26/2019 The Camera Verses the Human Eye
1/10
The
Camera
Versus
the
Human
Eye
Nov17,2012 RogerCicala
Thisarticlewasoriginallypublishedasablog.PermissionwasgrantedbyRogerCicalatore
publishthearticleontheCTIwebsite.Itisanexcellentarticleforthosepolicedepartments
consideringtheuseofcameras.
ThisarticlestartedafterIfollowedanonlinediscussionaboutwhethera35mmora50mmlens
onafullframecameragivestheequivalentfieldofviewtonormalhumanvision.Thisparticular
discussionimmediatelydelvedintotheopticalphysicsoftheeyeasacameraandlensan
understandablecomparisonsincetheeyeconsistsofafrontelement(thecornea),anaperture
ring(theirisandpupil),alens,andasensor(theretina).
Despite
all
the
impressive
mathematics
thrown
back
and
forth
regarding
the
optical
physics
of
theeyeball,thediscussiondidntquiteseemtomakesenselogically,soIdidalotofreadingof
myownonthetopic.
Therewontbeanydirectbenefitfromthisarticlethatwillletyourunoutandtakebetter
photographs,butyoumightfinditinteresting.Youmayalsofinditincrediblyboring,soIllgive
youmyconclusionfirst,intheformoftwoquotesfromGarryWinogrand:
7/26/2019 The Camera Verses the Human Eye
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Aphotographistheillusionofaliteraldescriptionofhowthecamerasawapieceoftimeand
space.
Photographyisnotaboutthethingphotographed.Itisabouthowthatthinglooks
photographed.
Basicallyindoingallthisresearchabouthowthehumaneyeislikeacamera,whatIreally
learnedishowhumanvisionisnotlikeaphotograph.Inaway,itexplainedtomewhyIsooften
findaphotographmuchmorebeautifulandinterestingthanIfoundtheactualsceneitself.
TheEyeasaCameraSystem
Superficially,itsprettylogicaltocomparetheeyetoacamera.Wecanmeasurethefrontto
backlengthoftheeye(about25mmfromthecorneatotheretina),andthediameterofthe
pupil(2mmcontracted,7to8mmdilated)andcalculatelenslikenumbersfromthose
measurements.
Youllfindsomedifferentnumbersquotedforthefocallengthoftheeye,though.Someare
fromphysicalmeasurementsoftheanatomicstructuresoftheeye,othersfromoptometric
calculations,sometakeintoaccountthatthelensoftheeyeandeyesizeitselfchangewiththe
contractionsofvariousmuscles.
Tosummarize,though,onecommonlyquotedfocallengthoftheeyeis17mm(thisis
calculatedfromtheOptometricdioptervalue).Themorecommonlyacceptedvalue,however,
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is22mmto24mm(calculatedfromphysicalrefractionintheeye).Incertainsituations,the
focallengthmayactuallybelonger.
Sinceweknowtheapproximatefocallengthandthediameterofthepupil,itsrelativelyeasyto
calculatetheaperture(fstop)oftheeye.Givena17mmfocallengthandan8mmpupilthe
eyeballshouldfunctionasanf/2.1lens.Ifweusethe24mmfocallengthand8mmpupil,it
shouldbef/3.5.Therehaveactuallybeenanumberofstudiesdoneinastronomytoactually
measurethefstopofthehumaneye,andthemeasurednumbercomesouttobef/3.2tof/3.5
(Middleton,1958).
Atthispoint,bothofyouwhoreadthisfarprobablyhavewonderedIfthefocallengthofthe
eyeis17or24mm,whyiseveryonearguingaboutwhether35mmor50mmlensesarethe
samefieldofviewasthehumaneye?
Thereasonisthatthemeasuredfocallengthoftheeyeisntwhatdeterminestheangleofview
ofhumanvision.Illgetintothisinmoredetailbelow,butthemainpointisthatonlypartofthe
retinaprocessesthemainimagewesee.(Theareaofmainvisioniscalledtheconeofvisual
attention,therestofwhatweseeisperipheralvision).
Studieshavemeasuredtheconeofvisualattentionandfoundittobeabout55degreeswide.
Ona35mmfullframecamera,a43mmlensprovidesanangleofviewof55degrees,sothat
focallengthprovidesexactlythesameangleofviewthatwehumanshave.Damnifthatisnt
halfwaybetween35mmand50mm.Sotheoriginalargumentisended,theactualnormallens
ona35mmSLRisneither35mmnor50mm,itshalfwayinbetween.
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TheEyeisNotaCameraSystem
Havinggottentheanswertotheoriginaldiscussion,Icouldhaveleftthingsaloneandwalked
awaywithyetanotherbitoffairlyuselesstriviafiledawaytoamazemyonlinefriendswith.But
NOOoooo.WhenIhaveabunchofworkthatneedsdoing,IfindIllalmostalwayschooseto
spendanothercoupleofhoursreadingmorearticlesabouthumanvision.
Youmayhavenoticedtheabovesectionleftoutsomeoftheeyetocameraanalogies,because
onceyougetpastthesimplemeasurementsofapertureandlens,therestofthecomparisons
dontfitsowell.
Considertheeyessensor,theretina.Theretinaisalmostthesamesize(32mmindiameter)as
thesensoronafullframecamera(35mmindiameter).Afterthat,though,almosteverythingis
different.
The
retina
of
a
human
eye
Thefirstdifferencebetweentheretinaandyourcamerassensorisratherobvious:theretinais
curvedalongthebacksurfaceoftheeyeball,notflatlikethesiliconsensorinthecamera.The
curvaturehasanobviousadvantage:theedgesoftheretinaareaboutthesamedistancefrom
thelensasthecenter.Onaflatsensortheedgesarefurtherawayfromthelens,andthecenter
closer.Advantageretinaitshouldhavebettercornersharpness.
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Thehumaneyealsohasalotmorepixelsthanyourcamera,about130millionpixels(you24
megapixelcameraownersfeelinghumblenow?).However,onlyabout6millionoftheeyes
pixelsarecones(whichseecolor),theremaining124millionjustseeblackandwhite.But
advantageretinaagain.Bigtime.
Butifwelookfurtherthedifferencesbecomeevenmorepronounced
Onacamerasensoreachpixelissetoutinaregulargridpattern.Everysquaremillimeterofthe
sensorhasexactlythesamenumberandpatternofpixels.Ontheretinatheresasmallcentral
area,about6mmacross(themacula)thatcontainsthedensestconcentrationofphoto
receptorsintheeye.Thecentralportionofthemacula(thefovea)isdenselypackedwithonly
cone(colorsensing)cells.Therestofthemaculaaroundthiscentralcoloronlyareacontains
bothrodsandcones.
Themaculacontainsabout150,000pixelsineach1mmsquare(comparethatto24,000,000
pixelsspreadovera35mmx24mmsensorina5DMkIIorD3x)andprovidesourcentralvision
(the55degreeconeofvisualattentionmentionedabove).Anyway,thecentralpartofour
visualfieldhasfarmoreresolvingabilitythaneventhebestcamera.
Therestoftheretinahasfarfewerpixels,mostofwhichareblackandwhitesensingonly.It
provideswhatweusuallyconsiderperipheralvision,thethingsweseeinthecornerofour
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eye.Thispartsensesmovingobjectsverywell,butdoesntprovideenoughresolutiontoread
abook,forexample.
Thetotalfieldofview(theareainwhichwecanseemovement)ofthehumaneyeis160
degrees,butoutsideoftheconeofvisualattentionwecantreallyrecognizedetail,onlybroad
shapesandmovement.
Theadvantagesofthehumaneyecomparedtothecameragetreducedabitasweleavethe
retinaandtravelbacktowardthebrain.Thecamerasendseverypixelsdatafromthesensorto
acomputerchipforprocessingintoanimage.Theeyehas130millionsensorsintheretina,but
theopticnervethatcarriesthosesensorssignalstothebrainhasonly1.2millionfibers,soless
than10%oftheretinasdataispassedontothebrainatanygiveninstant.(Partlythisis
becausethechemicallightsensorsintheretinatakeawhiletorechargeafterbeing
stimulated.Partlybecausethebraincouldntprocessthatmuchinformationanyway.)
Andofcoursethebrainprocessesthesignalsalotdifferentlythanaphotographycamera.
Unliketheintermittentshutterclicksofacamera,theeyeissendingthebrainaconstantfeed
videowhichisbeingprocessedintowhatwesee.Asubconsciouspartofthebrain(thelateral
geniculatenucleusifyoumustknow)comparesthesignalsfrombotheyes,assemblesthemost
importantpartsinto3Dimages,andsendsthemontotheconsciouspartofthebrainfor
imagerecognitionandfurtherprocessing.
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Thesubconsciousbrainalsosendssignalstotheeye,movingtheeyeballslightlyinascanning
patternsothatthesharpvisionofthemaculamovesacrossanobjectofinterest.Overafew
splitsecondstheeyeactuallysendsmultipleimages,andthebrainprocessesthemintoamore
completeanddetailedimage.
Thesubconsciousbrainalsorejectsalotoftheincomingbandwidth,sendingonlyasmall
fractionofitsdataontotheconsciousbrain.Youcancontrolthistosomeextent:forexample,
rightnowyourconsciousbrainistellingthelateralgeniculatenucleussendmeinformation
fromthecentralvisiononly,focusonthosetypedwordsinthecenterofthefieldofvision,
movefromlefttorightsoIcanreadthem.Stopreadingforasecondandwithoutmovingyour
eyestrytoseewhatsinyourperipheralfieldofview.Asecondagoyoudidntseethatobject
totherightorleftofthecomputermonitorbecausetheperipheralvisionwasntgettingpassed
ontotheconsciousbrain.
Ifyouconcentrate,evenwithoutmovingyoureyes,youcanatleasttelltheobjectisthere.If
youwanttoseeitclearly,though,youllhavetosendanotherbrainsignaltotheeye,shifting
theconeofvisualattentionovertothatobject.Noticealsothatyoucantbothreadthetext
andseetheperipheralobjectsthebraincantprocessthatmuchdata.
Thebrainisntdonewhentheimagehasreachedtheconsciouspart(calledthevisualcortex).
Thisareaconnectsstronglywiththememoryportionsofthebrain,allowingyoutorecognize
objectsintheimage.Weveallexperiencedthatmomentwhenweseesomething,butdont
recognizewhatitisforasecondortwo.Afterweverecognizedit,wewonderwhyintheworld
itwasntobviousimmediately.Itsbecauseittookthebrainasplitsecondtoaccessthe
memoryfilesforimagerecognition.(Ifyouhaventexperiencedthisyet,justwaitafewyears.
Youwill.)
Inreality(andthisisveryobvious)humanvisionisvideo,notphotography.Evenwhenstaring
ataphotograph,thebrainistakingmultiplesnapshotsasitmovesthecenteroffocusoverthe
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picture,stackingandassemblingthemintothefinalimageweperceive.Lookataphotograph
forafewminutesandyoullrealizethatsubconsciouslyyoureyehasdriftedoverthepicture,
gettinganoverviewoftheimage,focusinginondetailshereandthereand,afterafew
seconds,realizingsomethingsaboutitthatwerentobviousatfirstglance.
So
whats
the
Point?
Well,Ihavesomeobservations,althoughtheyrefarawayfromwhichlenshasthefieldof
viewmostsimilartohumanvision?.Thisinformationgotmethinkingaboutwhatmakesmeso
fascinatedbysomephotographs,andnotsomuchbyothers.Idontknowthatanyofthese
observationsaretrue,buttheyreinterestingthoughts(tomeatleast).Allofthemarebased
ononefact:whenIreallylikeaphotograph,Ispendaminuteortwolookingatit,lettingmy
humanvisionscanit,grabbingthedetailfromitorperhapswonderingaboutthedetailthats
notvisible.
Photographstakenatanormalangleofview(35mmto50mm)seemtoretaintheirappeal
whatevertheirsize.Evenwebsizedimagesshotatthisfocallengthkeeptheessenceofthe
shot.Theshotbelow(takenat35mm)hasalotmoredetailwhenseeninalargeimage,butthe
essenceisobviousevenwhensmall.Perhapsthebrainsprocessingismorecomfortable
recognizinganimageitseesatitsnormalfieldofview.Perhapsitsbecausewephotographers
tendtosubconsciouslyemphasizecompositionandsubjectsinanormalangleofview
photograph.
ThephotoabovedemonstratessomethingelseIvealwayswonderedabout:doesour
fascinationandloveforblackandwhitephotographyoccurbecauseitsoneofthefewways
thedensecone(coloronly)receptorsinourmaculaareforcedtosendagrayscaleimagetoour
brain?
Perhapsourbrainlikeslookingatjusttoneandtexture,withoutcolordatacloggingupthat
narrowbandwidthbetweeneyeballandbrain.
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Likenormalangleshots,telephotoandmacroshotsoftenlookgreatinsmallprintsorweb
sizedJPGs.Ihavean810ofanelephantseyeandasimilarsizedmacroprintofaspideron
myofficewallthatevenfromacrosstheroomlookgreat.(Atleasttheylookgreattome,but
youllnoticethattheyrehanginginmyoffice.Ivehungtheminacoupleofotherplacesinthe
houseandhavebeentactfullytoldthattheyreallydontgowiththelivingroomfurniture,so
maybetheydontlooksogreattoeveryone.)
Theresnogreatcompositionorotherfactorstomakethosephotosattractivetome,butIfind
themfascinatinganyway.Perhapsbecauseevenatasmallsize,myhumanvisioncansee
detailsinthephotographthatInevercouldseelookingatanelephantorspiderwiththenaked
eye.
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Ontheotherhand,whenIgetagoodwideangleorscenicshotIhardlyevenbothertoposta
websizedgraphicormakeasmallprint(andImnotgoingtostartforthisarticle).Iwantit
printedBIG.Ithinkperhapssothatmyhumanvisioncanscanthroughtheimagepickingout
thelittledetailsthatarecompletelylostwhenitsdownsized.AndeverytimeIdomakeabig
print,evenofasceneIvebeentoadozentimes,InoticethingsinthephotographIvenever
seenwhenIwasthereinperson.
Perhapsthevideomybrainismakingwhilescanningtheprintprovidesmuchmoredetailand
Ifinditmorepleasingthanthecompositionofthephotowouldgivewhenitsprintedsmall(or
whichIsawwhenIwasactuallyatthescene).
Andperhapsthesubconsciousscanningthatmyvisionmakesacrossaphotographaccounts
forwhythingsliketheruleofthirdsandselectivefocuspullsmyeyetocertainpartsofthe
photograph.Maybewephotographerssimplyfiguredouthowthebrainprocessesimagesand
tookadvantageofitthroughpracticalexperience,withoutknowingallthescienceinvolved.
ButIguessmyonlyrealconclusionisthis:aphotographisNOTexactlywhatmyeyeandbrain
sawatthescene.WhenIgetagoodshot,itssomethingdifferentandsomethingbetter,like
whatWinograndsaidinthetwoquotesabove,andinthisquotetoo:
Youseesomethinghappeningandyoubangawayatit.Eitheryougetwhatyousaworyouget
somethingelseandwhicheverisbetteryouprint.
About
the
author:RogerCicalaisthefounderofLensRentals.Thisarticlewasoriginally
publishedhere.
Image
credits:myeyeupclosebymachinecodeblue,Nikhseyethroughcameraseyefrommy
eyesforyoureyes:)byslalit,SchematicoftheHumanEyebyentirelysubjective,Mylefteye
retinabyRichardMasoner/Cyclelicious,Chromaticaberration(sortof)bymoppet65535