国外权威摄影杂志The Ultimate Beginners Introduction to Exposure.docx
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国外权威摄影杂志TheUltimateBeginnersIntroductiontoExposure
odaywe’llbewalkingthroughtheconceptofexposure,rightfromthebeginning.Thisarticlewon’tbefuddleyouwithcomplicatednumbersandjargon,butitwillhelpyoufeelmoreconfidentwithyourphotographyandunderstandacoreconceptthat’savitalpieceofthephotographyjigsaw.
Preface
Ibelievetherearethreedifferenttypesofphotographers:
technical,artistic,andthosethathavegoodartisticvisionandcombinestechnicalknowledgetoachievethatvision.Noneisrightorwrong.Allarejustdifferentinapproachandexecution.
ThefirstphotographerIevermentoredwasanamazinglyartisticwoman.Shehadaknackforseeingthingslikeno-onehadeverseen.Shewasalsoavoraciouslearnerandsheaskedmeforhelponthetechnicalsideofthingstomakeherselfbetter.
Istartedspoutingallthenumbers,allthetheories,andallthescience.Theresponse?
“Whoa,Myminddoesn’tthinkthatway”.Andshewasright.Peoplewhoareveryartisticthinkdifferently,theirbrainscomputethingsdifferentlythansomeonethatthinksmoreanalyticallyandscientifically.
Ineededtochangemyteachingmethodstofitthewayshethinks.SothatiswhatIwilldowiththisarticle–explainthetechnologywithoutthemathandnumbersandtheories.
Bytheway,thatstudentisnowoneofthetopportraitphotographersinherstate(heck,Ithinkeverywhere)andIcouldn’tbemoreproudofher.Sheinspiresmeeveryday.
TheTriangleofExposure
Therearethreemainingredientstogoodexposure:
Aperture,ShutterSpeed,andISO.Okay,yesIsaidnomath,butIwilluseabitofgeometrytoillustratetheeffectthatthethreecomponentsofexposurehaveoneachother.IamcertainlynotthefirsttothinkofthisasatrianglebutIdothinkitisthebestrepresentation
Thinkofaperfectexposureasaperfecttriangle–alltheanglesareequal,allthesidesareequal.Nowifyouchangejustonepartofthatexposureortriangle,itisnolongerperfectsoyouwillneedtochangeanotherpointoftheexposureortriangleanequalbutoppositeamounttomakethattriangleandthereforetheexposureperfectagain
Asyoucansee,alltheelementsofexposurehaveaneffectontheothers–sowiththatinmindweneedtoknowthehowsandwhysofallthedifferentelementstobestunderstandhowtogetbothagoodexposureandthedesiredresultsinourphotographs.
Nowyoumayask;“Whydoweneedtohavealldifferentsettingsforexposure.Whyisn’ttherejustone?
”Well,inthelastcenturyonalmostallofthepointandshootcameras,that’sthewayitwas.Theaperturewasfixed,aswastheshutterspeedand–eventhoughyoucouldbuyfilmthatwasdifferentISOs–therewasusuallyonlyonerecommendedforthatparticularcamera.Butitwasverylimiting.
Sincethosecameraweresetupforanaveragescene,youeithershotanormaldaylightsceneor(ifyoupoppedonaflash)anindoorscene.Forgetaboutshootinganaturallightsunsetornightshot.Forgetaboutstoppingactionofaracecar.Youwerestuckwithwhatyouhad.
Nowwewanttobemoreartisticinourphotosandwewantmorecontroloverwhatweshoot.Sotoachievethatartisticandtechnicalcontrol,weneedtoknowaboutthedifferentsettingswecanuseandwhyweusethem.
Solet’sstartwithAperture
Aperture
Apertureisacircularopening(somewhat)inourlensthatisadjustablefromaverysmallcircletoalmostaslargeasthelensitself.Weadjustittoletmoreorlesslighthitthedigitalsensororfilm.Thinkofwindowblindsasyouraperture,andthewallinyourroomoppositetheblindsisyoursensororfilm.Asweopentheblinds,morelightcomesthroughandwecanseethewallbehindusgetbrighterandbrighter
Likewise,asweopenuptheapertureonourlens,wegetmorelightonoursensororfilm.
TheopeningofourlensorApertureisexpressedinfstopsandhereisaverytypicalrangeoffstops:
Youmaysay;nowwaitaminute,whydothelargestopeningshavethesmallestnumbers?
Well,thinkofitthisway;imaginethenumberasthebottomnumberofafraction.Soifwehavef4thatwouldbe1/4andf8wouldbe1/8and1/4islargerthan1/8.Okay?
Okay.
TheAperturenumbersIhaveaboverepresent“WholeSteps”oflightfromonetothenext.Whatisawholestep?
Awholesteprepresentsthedoublingorhalvingofthelightthroughthelens.Sof1.4willletintwiceasmuchlightasf2.0.f2.0twiceasmuchlightasf2.8orwecanalsosaythatf2.8ishalfasmuchlightasf2.0.
Onyourlens,youmayseenumbersinbetweenthenumbersabove.Thoserepresenteither1/3orΩstops(dependingonyourcameramodel)sowecanfinetunebeyondjustwholestops.
Sowehaveallthesedifferentaperturesavailabletoletindifferentamountsoflight.WhyshouldIcareandwhyshouldchooseoneovertheother?
Thisiswheretheartistsidecomesin–tohelpmakethosedecisions.ThefollowingaredifferentartisticstyleswecanuseandthedifferentpartsofanimagethatApertureeffects.
DepthofField
Whenwelookatanimage,thereisapartthatisinperfectfocusandthentherearepartsthatbegintobeoutoftherangeoffocus.YoucanhaveasmallDepthofField(DOF)whereonlyyoursubjectisinfocus,oryoucanhaveadeepDepthofFieldwherepracticallyeverythingisinfocus–orreallyanywhereinbetween.
Depthoffieldisdeterminedbythreethings;aperture(fstop),distancetoyoursubject,andlensfocallength(50mm,200metc),withAperturehavingaprofoundeffectonDOF.Let’sseewhatanimagelookslikejustvaryingtheapertureandleavingalonetheothertwoparts:
Photographersthatshootportraitsusuallyuselargerapertures(lownumbers)forashallowDOFtohighlightandisolatetheirsubjects.LandscapephotographersusuallyusesmallaperturestohaveaverydeepDOF,allthewayfromtheforegroundtothebackground.
Asusual,thereareexceptionstotheserulesandthatisruledbythephotographer’sartisticideasandvision.
ShootinginLowLight
Aswesawwiththewindowblindsexample,openingupourapertureallowsmorelighttohitoursensororfilm.Sowhenshootinginlowlight,itmayhelptoopenupyouraperturetomakeiteasieronourothertwosidesoftheexposuretrianglewhichIwillexplainmorelaterinthosesectionsonShutterspeedandISO.
ShutterSpeed
Shutterspeedcontrolshowlongthelightcomesthroughouraperturetoourdigitalsensororfilm.Thelongerthetime,themorelightwillhittheirsurface.Thatistechnicallywhatshutterspeeddoes.
Artistically,shutterspeedcontrolsmotion.Whetherwewanttofreezemotionorshowmotion,shutterspeedistheportionofexposurethatwillcontrolthataspect.
Shutterspeedsareexpressedinfractionofasecond1/8,1/125,1/1000etc.butinmostmoderndigitalcamerasyoumaynotseethe1/expressed(buttheystillarefraction).Youmostlikelyseetheaboveexpressedas8,125,1000etc.
Thefirstconsiderationwehavetothinkaboutis:
canwehand-holdthecameraatthisshutterspeedandnothavetheslightmovementofthecamerashowupinourimagemakingitblurryorlesssharp?
Mostpeopleareabletohandholdacameraintherangeof1/60thto1/200thwithanormallenson.Whenyouusetelephotolensesyoumayneedtouseafastershutterspeed.
Acommonruleofthumbthesedaysistotakeyourfocallengthofyourlensandshootatleastasfastasthat.Ifyouhavea300mmtelephotolens,yourminimumshutterspeedshouldbe300(1/300or1/320asiscommon)Ifyoucannothand-hold,youmustuseatripod.
Beyondthatwecannowmakeanartisticjudgment–dowewanttostopaction,orshowmovement?
Andthisisajudgmentyouneedtomake.Sometimeswemaywanttofreezeoursubjectandkeepitsharpandclear.Othertimeswewantsomebluronthesubjecttogivetheviewertheimpressionthattheobjectismovingorisatspeed.
Inthisimageofthetrolleydowntown,inthefirstexampleweusedahighshutterspeedtostopthetrolleyasitmovedpastthebuildings(whichmaybewhatwewant).Butdoestheviewerreallyknowifthetrolleywasmovingpastorjuststoppedatthestation?
Inthesecondexampleweslowedtheshutterspeedwaydownto.3(3/10th)ofasecond.Nowtheviewercantellthatthetrolleyismovingatspeedpastthebuilding.
Againthisisthedecision you havetomake.Whatdoyouwanttoconvey?
Otherexamplesofusingahighshutterspeedtostopaction:
Stoppingtheflightofajetasitfliesby,oracarathighspeed.Freezingtheswingofthebatofabaseballplayer,ahighdiverinmidarcabovethewater.Againyouwoulduseahighshutterspeedtofreezethemovementoraction.
Intheimagebelow,aslowshutterspeedwasused(onatripod)tosilkenthewaterandshowmotion.Thesamecanbedoneforawaterfall,anoceanorafountain.
ISO
ISOisthesensitivitytolightofthesensororfilm.Thehigherthesensitivity,thelesstime(shutterspeed)orthelessamountoflight(aperture)needstohitthesensorforthecorrectexposure.Itisadjustableinmostcameras,inarangeofabout200to1600.Highendcamerascangowaybeyondthat.
WeuseISOtohelpusachievewhatwewanttodowiththeothertwosidesofexposure;ApertureandShutterSpeed.Youmayask;Whydon’twejustusethemostsensitiveoneandforgetaboutit?
WellbecausethedownsideofhigherISOisthatitincreasesthenoiseorgraininourimages.Thiscansometimesmaketheimagelooksobadthatitbecomesunusable(oratleastun-printableatadecentsize).
SoourobjectiveistousethelowestISOpossible,butbalancingthattowhatwewanttoachieve.
Shootingoutdoorsonsunnyorevenslightlyovercastdayswe