subscribe 0330 333 1113I http://www.amateurphotographer.co.ukI 16 May 2020 61
Tech Talk
ofessorNewmanon...
BobNewmanis currentlyProfessorof ComputerScienceat theUniversityof Wolverhampton.Hehasbeenworkingwiththedesignanddevelopmentof
high-technology equipment for 35 years and two of his products have won innovation awards. Bob is also a camera nut and a keen amateur photographer
E x posu re a nd
ightness
O
neof theprincipal
conceptsof
photographyis
exposure.It’soneof
thefirstpiecesof jargonthata
photographeradoptsto denote
membershipof theclub.Showa
non-photographera shotthat
lookstoodarkandthey’lllikelysay
‘that’stoodark’.Showit to a
photographerandthey’llsay‘that’s
underexposed.’Oddly,thelay
personis quitelikelyto becorrect,
becauseunderexposureis onlyone
possiblecauseof a photolooking
toodark.Thispointis oftenloston
thosephotographersthat
erroneouslythinkthatexposure
meanshowlightordarkthefinal
imagelooks.So,whatdoes
exposuremean?
Technically,it is theluminous
energydensityat thefocalplane.
Let’sunpickthata little.Luminous
meansnotjustlight,butmore
specificallyvisiblelight.So
luminousenergyis
electromagneticenergy,whichis
withinthevisiblespectrum.The
conceptof luminositygoesa little
further,andweightsthedifferent
partsof thespectrumaccordingto
howvisibletheyaretothehuman
eye.Thefunctionusedis called
the luminosity function (below).
Atthisstageit’sworththinking
aboutthephysicalrelationship
betweentheconceptsof energy
andpower.Poweris therateof
applicationof energy,thusa
high-powerenginecangeneratea
lotof energyin a shorttime.A
low-powerenginecanstill
generatethesameamountof
energy,butneedslongerto do it.
Luminousenergy
Theunitof energyis thejouleand
thatof power,thewatt.The
correspondingunitforluminous
poweris thelumen.Thereis no
standardunitof luminousenergy,
sowemultiplythelumenratingby
thetimeforwhichit is usedtogive
thelumensecond.Intensityof light
(powerdividedbyarea)is
importantformanyreasons,sohas
it’sownunit,thelux.Thusweend
upwiththeunitof exposure,the
lumensecond,whichgivesusthe
luminousenergydensity,in the
caseof exposure,at thefocalplane
of thecamera.A photographercan
controltheluminouspowerdensity
bychangingtheaperture,which
regulateshowmuchof thelight
fromthesceneis allowedtopass,
andbychangingtheamountof
lightincidentonthesubject.The
luminous energy density (exposure)
canbecontrolledeitherbychanging
thepowerdensityortheamountof
timethatthelightis allowedto
impingeonthefocalplane.This
leadsto a conclusionthe
photographersalreadyknow,that
exposurecanbecontrolledbythe
sceneillumination,theapertureand
theexposuretime.
So,whatis therelationshipof
exposuretohowlightanddarkthe
finalimageis?Thatpropertyis
controlledbywhatwedowiththe
exposure.If weconsiderjust
positiveprocesses(negative
processescontaina printingstage,
whichis criticalin thisrespect)we
canchangewhattheimagelooks
likebychangingtheexposure,by
changingthemediumthatweuse
torecordthesceneandby
changinghowwedevelopor
processthatmedium.A digital
camerahasonerecordingmedium,
soouronlyoptionsin thatcaseare
toaltertheexposureorthe
processing.Thepropertythat
encapsulatesthebehaviourof the
processing,in howit translates
exposuretothefinalappearanceof
theimage,is calledISO.Toreturn
tothepointraisedin thefirst
paragraph,animagemaybetoo
darkbecauseit hasbeen
processedtobetoodark, as well as
beingunderexposed.
Lightnessdefined
Giventhatexposuredoesnot
describehowlightordarkthe
imagelooks,whatwordshouldwe
use?Myfavouredcandidateis
‘lightness’,whichis the‘L’ in theLab
colourspace,whichformsthebasis
of mostcolourprocessing.The
wordhaslinksfromthe1920sby
meaningpositionona perceptual
scalefromwhatanobserversees
as‘black’to‘white’,sohashistory
onitsside,aswellasmeaning
exactly what is required in this case.
Underexposureisonlyonecauseofanimagelooking
too dark, the term lightness would carry more meaning
TheCIEluminosityfunction
defineshowmucheach
wavelengthoflightcontributes
to luminous power
Subscribenow
fromonly
£1.9 6
per issue
*
Yourfavourite
magazine
deliveredstraight
toyourdoor
every week...
magazinesdirect.com/55AW
0330333 1113
Quotecode: 55AW
MondaytoSaturday
from 8am to 6pm (UK time)
Offercloses
31 December2020.
Forfulldetailsvisit
http://www.magazinesdirect.com.
