Introduction to Digital Resolution · Theoptimum pixel resolution of an image is dependent on its...

Introduction to Digital

Resolution

2011

Copyright Les Walkling 2011

Adobe Photoshop screen shots reprinted with permission from Adobe Systems Incorporated.

Version 2011:02

CONTENTS

• PixelsofResolutionandLevelsofTonality 3

• DigitalResolution:HowmanyPixelsdoIneed? 4

• BitDepth:HowmanyLevelsdoIneed? 5

•LevelsofBrightness 6

• CameraResolution 7

• ImageResolution 8

• MonitorResolution 9

• ColourGamutsandRGBProfiles 10

•Histograms 11

Copyright Les Walkling 2011 www.leswalkling.com 2/11


Pixels of Resolution and Levels of Tonality

Oureverydayperceptionofrealityleadsustointeractwiththeworldasthoughitwerecontinuous.Amagnifyinglensforexamplerevealsmoredetailthancanbereadilyseenwiththeunaidedeye.Ourexperienceoftraditionalanaloguephotographyhasreinforcedthisperception,andiswhyphotographyisknownasa‘continuoustone’mediumasop-posedtoa‘halftone’mediumsuchastheoffsetprintingofimagesinbooks,magazinesandnewspapersthatrearrangetheoriginalphotograph’s ‘continuousness’ intoaseaofhalftonedotsofinkonpaper.

Whilenotthesameasaprintingpress,digitalphotographyisalsoahalf tonemediumbecauseitreducesthesceneandits‘continuousresolution’intostepsofresolutionthatwecallPixelsandits‘continuoustone’intostepsofbrightnesscalledLevels.

Thesmallertheintervalsofresolution(pixelation)andbrightness(posterization)thatanimageisdigitisedinto,themoreitsappearancewillmaintaintheillusionofcontinuous-ness.Theworldcanbedigitisedbyphotographingitdirectlywithadigitalcamera,orbyscanningphotographicnegatives,slidesorprints.Butoncedigitisedtheimageispixelat-edandposterizedanditwillremainthatway.Thereforechoosingthecorrectimagepix-elationandposterizationiscriticalformaintainingtheillusionofanaloguephotography.

Thisraisestwoimportantquestions:

Thetypeofdigitalcameraanditssettingscontroltheseoutputvalues.

How many Pixels do I need?

How many Levels do I need?

Levels of brightness

Pixelation

Pixel resolution

Posterization

Digital Resolution


Digital Resolution: How many Pixels do I need?

Thesensorinadigitalcameraisratedintermsofthenumberofpixelsitdividestheim-ageinto.Aonemegapixel(mpx)camerathereforewillcaptureapproximatelyonemillionpixelswhichwhenprocessedintoanRGBfilewillproducea3megabyte(mb)image(1mbRed+1mbGreen+1mbBlue=3mbRGB).Canon’sEOSDSLRrangeincludes:

Internet/Monitor DistributionTheinternetonlyrequiresverylowresolution,72pixelsperinch(ppi)images,becausethatisthetypicalresolutionofthelowresolutionmonitorsthatmakeupmostofthein-ternet.Forexample,a6”x4”(15cmx10cm)image@72ppi=364.5kbwhichisapproxi-matelyathirdofamegabyte(mb).

Print DistributionWhenprintingdigitalimagesdifferentfilesizesarerequiredforoptimumqualityonvari-ousprinters.Typicalexamplesarelistedbelow.Someprinters,suchastheLambdadigitalphotographicprinterfoundinmanyprofessionallabswillacceptimagesateither200ppior400ppi,thelateronlyrequiredforthehighestresolutionprintingofblacktextonwhitebackgrounds.

Noritsu Photo MiniLab Epson R2400 Inkjet Printer6”x4”@400ppiprint=11.0MBRGB 6”x4”@360ppiprint=8.9MBRGB8”x10”@400ppiprint=36.6MBRGB 8”x10”@360ppiprint=29.7MBRGB 16”x20”@360ppiprint=118.7MBRGB

Kodak Pegasus LED Photo Printer Lambda Digital Photo Printer8”x10”@250ppiprint=14.3MBRGB 8”x10”@200ppiprint=9.16MBRGB20”x32”@250ppiprint=114.4MBRGB 40”x60”@200ppiprint=274.7MBRGB

Morepixelswillsometimesberequiredthanacameracanproduce.Inthiscasethefileswillneedtoberesizedand/orprintedatalowerthanoptimumresolution.Mostdigitalcamerafilescanusuallybedoubledinfilesizewithminimallossofquality,especiallyinRAWmode.Greaterenlargementsmustbemadewithcareand/ortheresultingimagesneedtobeviewedatagreaterdistancetoavoidanobviousdecreaseinimagequality,suchasloweredsharpness(notascrisp)androughertones(notassmoothandcontinuous).Inpracticeadigitalfilecanoftenbeconsiderablyenlargedandstillappearacceptable.

1DsMarkIII 21.1 Megapixels 5.0 fps 5616x3744px 63.3mbRGB1DMarkIII 10.1 Megapixels 10.0 fps 3888x2592px 30.3mbRGB5DMarkII 21.0 Megapixels 3.9 fps 5616x3744px 63.0mbRGB50D 15.1 Megapixels 6.3 fps 4752x3168px 45.3mbRGB450D 12.2 Megapixels 3.5 fps 4272x2848px 36.6mbRGB

Digital Resolution


Bit Depth: How many Levels do I need?

Digitalphotographyrequires,ataminimuman8bitperchannelenvironmentwhere256discrete levelsorstepsof tone (28=256)describe theentire tonal range fromblack towhite.Ifthestepsoftonearesmallenoughincrementswedon’tnoticethemasdiscretestepsorjumpsoftoneandthedigitalimagewillthenpreservetheillusionofsmoothcon-tinuouscolour,andundernormalviewingconditionswillbe indistinguishable fromanoriginalcontinuoustone‘photograph’.

A24-bitcolourimage=three8-bitchannels.One8-bitchanneldescribestheimage’sREDvalues,theseconddescribesitsGREENvalues,andthethirddescribesitsBLUEvalues.

Mostmonitors/printers are alsodesigned toworkwith8-bitsper channel. Inorder toachieveanoptimum24-bitRGBfile,digitalcamerasrecordthesceneinmorethan8bitsperchannel.Thehigherthecamera’sbitdepththegreaternumberof levelsoftonetheimageiscapturedin.Ahigherbitdepthresultsinsmoothertonaltransitionsandlessob-viousposterization.ACanon1DsMkIIIforexamplecapturesin14-bits(16384Levels)butwhenprocessedin-camerareducesthebitdepthtoproduceanoptimum8-bitsperchannel.Theaimof‘normalprocessing’isanimagewhoseposterizationwillnotbevisiblewhenviewedonamonitororinaprint.Forexample:

224=256levelsREDx256levelsGREENx256levelsBLUE=16.77millioncolours.

4 bits per channel (16 levels) 8 bits per channel (256 levels)

8 bits RED + 8 bits GREEN + 8 bits BLUE = 24 bit Colour

Digital Resolution


Digital Resolution

Levels of Brightness

Thesimplestdigitalimageisa1bitimage(21)whichdividestheoriginalimageintoonlytwolevelsofbrightness;blackandwhite.Thebitdepthdeterminesthemaximumnumberoflevelsorstepsofbrightnessfromblacktowhiteintheimage.Forexample:

1bitimage=21=2levelsofbrightness(=blackorwhite) 2bitimage=22=4levelsofbrightness 3bitimage=23=8levelsofbrightness 4bitimage=24=16levelsofbrightness 5bitimage=25=32levelsofbrightness 6bitimage=26=64levelsofbrightness 7bitimage=27=128levelsofbrightness 8bitimage=28=256levelsofbrightness(=theillusionofcontinuoustone)

Adigital image isaposterized image.Therefore for thehighestpossible imagequality,intermsofmaintainingtheillusionofcontinuoustoneandcolour,youwanttoendwith,ratherthanstartwith,8-bitsperchannel.Todothisyouhavetodigitise(scanner/camera)yourimagesintomorethan8-bitsperchannel,sothatafterimageeditingyoustillhaveenoughbitdepthlefttomaintainthisillusion.Forexamplea16-bitworkflow(216)resultsin65,535levelsoftoneperchannelwhichcansupportextensiveeditingwithoutvisibleposterization.Photoshopoffersextensivesupportfor16-bitimages,thoughitscontrolsstillonlyeditwith8-bitresolutionandits16-bitInfoPaletteonlydisplaystruncated16-bitvalues(0to32,768).Anotherissueisthat16-bitresolutiondoublesthefilesizewhichincreasesstoragerequirementsandsignificantlyslowsdownallpixeleditingoperations.

Increasing levels of brightness decreases levels of visible posterization


Camera Resolution

DigitalSingleLensReflexcamerasreplacefilmwithagridorarrayofphoto-sites(pixels).Thesceneprojectedbythelensisdigitisedaccordingtothenumberofphoto-sitesinthearray.Forexamplea6megapixel(mpx)camerawilldividetheimageinto6millionpixelsperchannel.A6mpxcamerawillthereforeresultinan18megabyte(mb)RGBimage.

Eachphoto-sitecanonlyrecordtheamount(orlevel)oflight,notitscolour.Inordertoproduceafullcolourimage,eachpixelinthearraymustrepresentitscorrespondingpointinthesceneaccordingtoitsoriginalred,greenandbluebrightness.

ProcessingtheRawfileincludesinterpolatingeachphoto-site’s‘missing’GBorRBorRGlevels fromthesurroundingphoto-sites .Unfortunately this ‘demosaicing’also leadstodigitalartifactsandaberrationsthatmayneedtobecorrected.Highermegapixelchipscanbeenlargedfurtherbeforetheartifactsbecomevisible,butallchipsresultinartifacts.DarkcurrentnoiseandJPEGcompressionartifactsfurtherreduceimageresolution.

Thefinalpicturequalitydependsonmanyfactorsincludingthearray’ssensitivitytolight,itssignal tonoiseratio, theheat itgeneratesand/orhowwell it iscooled, thecamera’sproprietyimageprocessingalgorithms,andthenumberofphoto-sites(megapixels).

Digital Resolution

Buteachphoto-siteiscoveredbyonlyaredorgreenorbluefilter thatmeasuresonly the redorgreenorbluebrightnessofthelightfallingonit.Atypicalarrayalsohas twiceasmanygreenas redorbluefilteredphoto-sites.While different manufacturers arrange the pixelarrayandfiltrationindifferentconfigurationsaccordingtotheirproprietydesign,thecapturedfilealwayshastobeprocessedintoanRGBimage.

Dark current noise JPEG compression artefacts

Bayer Filter Array


Image Resolution

Theoptimumpixelresolutionofanimageisdependentonitssubjectmatterandrequiredopticalresolution,outputmedium,andtheintendedprintviewingdistance.

Subject Matter: Lowcontrast and/ordiffuse subjects, suchasmist, clouds andoutoffocusareascanbesatisfactorilyrenderedatlowerresolutionsthansharplyfocusedhighlydetailedimages.Thefollowingimagewasrenderedatthesamephysicalsizebutdifferentpixelresolutions.Photoshop’stypetoolwasusedtoindicateeachfile’sresolution.Allim-ageswereprintedattheprinter’shighestqualityandthesamesizeonglossypaper.

Pixels per Inch (ppi): Refers to the resolution of the file. Crisp black text on awhitebackgroundisagoodtestforthehighestrequiredppiresolutionforanimage.Renderingandprintingtextathigherppiresolutionswillrevealhowmanypixelsarerequiredforcleansharplookingtext.Therenderingofpictorialsubjectslikeskin,sky,andcloudswillnormallynotbeimprovedbyrenderingatahigherppiresolution.Howeversomeprinterdriversexpecttoreceiveapreciseppivalue,andtheywillautomaticallyresizeimagestothatrequiredppi.Thiscanresultindecreasedopticalresolutiondespitehigherppiresolu-tion,andcanonlybeverifiedbycriticalprintingtests.

Digital Resolution

Scaled to 50ppi and printed at 2880dpi Scaled to 100ppi and printed at 2880dpi

Scaled to 200ppi and printed at 2880dpi Scaled to 360ppi and printed at 2880dpi


Monitor Resolution

Themonitor’smaximumresolutionequals itsphysical sizemultipliedby itspixelden-sity.

PixelDimensions:Fixedsizeobjects(suchasPhotoshoppalettes)willappearmuchlarger(approximatelyfourtimeslarger)onasmall640x480pixeldesktopcomparedtoalarger1280x1024pixeldesktop.Largerpixeldimensions,notaphysicallylargerscreenprovidesmoredesktopspaceforyourimagerelativetoPhotoshop’spalettes,menusandtoolbars.Smallerpixeldimensionsresultinacrowdedandlessefficientdesktop/workspace.

Thecomputer’sDisplayControlPanel (PC)orDisplaysPreferencePane (Mac)will in-dicatetherangeofmonitorresolutionsitsvideocardcansupport.Setthemonitortoaminimumof24bitcolourandthehighestpixeldimensionsthatstillallowseasynaviga-tionofthedesktopicons,toolsandmenus.WhilemanyLCDpanelsoperatebestattheir‘native’ormaximumpixelresolution,verylargescreens(30”)mayrendermenuitemsanddialogueboxestoosmall(attheirnativeresolution)toreadwhenseatedatacomfortableworkingdistance.SittingclosertomoreeasilyreadandinteractwithdialogueboxesandmenuitemsmayresultinfrequentheadturningandOccupationalHealthandSafetyis-sues.Theseconcernscanbereducedonscreenslargerthan24”byworkingonimagesthataresmallerthantheentirescreen,whileusingthesurroundingareasforpalettes.

Digital Resolution

Awidescreen1440X900pixelresolutionpro-videsampleroomforPhotoshoppaletteswith-outencroachingontheimage.At800x600pix-elsthescreeniscrowded.Aresolutionof1920x1200pixelsisoftenconsideredoptimumforimageeditingonlarge(23”to30”)screens.

1440 x 900

800 x 600


Colour Gamuts and RGB Profiles

DigitalSLRcamerasarecapableofrecordingverylargecolourgamuts,oftenequaltoorgreaterthanthatcapturedbyfilm.ForexamplethegamutofaCanonEOS1DsMkIIIisverysimilartothegamutofKodakEktachromeEPN(ISO100).

Theseareverylargegamutscapableofaccuratelycapturingmanyofthemostsaturatedcolourswe can see (within the visual locus).They also substantially exceed the colourgamutofallmonitorsandprinters.Thereforeitistemptingtosavecamerafilesinacolourspace(gamut)substantiallysmallerthanitsoriginalgamut.Thiswillpotentiallyresultinmanysaturatedcoloursbecomingclippedand lost.Forexample, selectingsRGBas thein-cameracolourspace,acolourspaceroughlyequivalenttoanuncalibratedPCmonitor,isfineifyouroutputisdestinedfortheinternet,butitwouldnotberecommendedforimagesfullofsaturatedcoloursthataretobeprintedonawidegamutinkjetprinter.

ThedangerofcompressingacolourfulimageintosRGBisthatimportantcolourdistinc-tionsmightbe lost. It isoftenbetter to save the image ina largecolourspace likeProPhotoRGB.

Alltheoriginalcolourdifferenceswillthenbefaithfullypreservedandcanbeconvertedintoprintablecoloursatthetimeofprinting.UnfortunatelyatthistimenodigitalcamerasupportswidegamutspaceslikeProPhotoRGB.HowevercapturingRAWdataandpro-cessinginaprogramlikeAdobeCameraRaw™willallowtheuseofProPhotoRGB.

Digital Resolution

CIE Lab: L* = 50%

1Ds Mk III

Ektachrome EPN

Yxy: L* = 50%

1Ds Mk III

Ektachrome EPN

Visual Locus

Pro Photo RGB Pro Photo RGB

sRGB


Digital Resolution

Histograms

Ahistogramisa‘pictureofapicture’thatgraphicallyrevealsthedistributionoftonesintheimage.Itmapsthetwoconstituentsofthedigitalimage,‘pixelsofresolution’,againstthe‘levelsofbrightness’representedbyeachpixel.Learningto‘read’theshapeofahisto-gramisanimportantskill,becauseitindicatesthenatureoftheimage,andaidsitsdiag-nosticandaestheticevaluation.

Ahistogramtypicallyconsistsofatleast256screenpixelwidecolumnsthateachrepre-sentadiscretelevelofbrightnessin8-bitnotation.Gapsinahistogram(emptycolumns)representa lossofcontiguous levelsof tone (<256).As thegaps increase, sodoes theposterizationintheimage.Smoothtonedareaslikeskyandskinwillfirstshowthisin-creasedposterizationasrougherorcoursertonaltransitions,andatworst,as‘banding’(distinctbandsofbrightnessacrosstheimage).

Reading HistogramsThehorizontal axis represents thelevelsofbrightness,wheretheshad-ows (darkervalues) are to the left,and the highlights (lighter values)are to the right. The vertical axisrepresents the relative percentageofpixelsateachlevelofbrightness.

Midtones

Shadows and Highlights


Digital Resolution

Understanding Unsharp Masking

Sharpening(andblurring)effectsareproportionaltothedegreeofenlargementofthefi-nalprint.Forexamplesharpeningthatappearsperfectlybalancedona100ppiscreenwillbecomeexcessivewhenenlargedina200ppiLambdaprint.Localeffectsareproportionaltothedistance(enlargement)overwhichtheyareprojected.Thereforeitisprobablywisetoapplythesesharpeningandblurringeffectsonlyoncethefinalimagesizehasbeenes-tablishedandatestprint(notthescreenimage)hasbeencarefullyevaluated.

GraduallyincreasetheRadiusuntiltheedgeshavebeenadequately‘outlined’,thenslow-lyincreasetheThresholdsettinguntiltheeffectjustbeginstowane.FinallyreducetheAmounttosuittheimage’scharacter.Thiswillgiveyouthebasicsharpeningsettingforthatimage.

AlwaysinspecttheeffectsoftheUnsharpMaskfilterat100%screenmagnification.Tolimitunwantedsharpeningartifactssuchascolourfringing,initiallyapplythefiltertoaduplicateimagelayerthenblendonLuminosity.Theduplicatelayerwillalsoallowthesharpeningtobelocallyinsertedviaalayermask.TheAdobePhotoshopsharpenandblurfilterscanalsobeappliedin16-bitcolourdepthwhichwillfurtherreduceunwantedim-ageartifactsandpromotesmoothertransitionsinthefinalprint.

SharpenLayerscanalsobeblendedonDarkenandLightentoseparatethedarkandlightoutlines,whichcanthenbeappliedatdifferentopacitiesthatbestsuittheimage.

AMOUNTis the strengthof thefilter. In theUnsharp Mask filter, 500%results in muchdarkerandlighteroutlines(halos)than100%.

RADIUS is thewidthof theoutline, inpixelseithersideoftheboundarybeingsharpened.

THRESHOLDisthenumberoflevelsoftonaldifferencethatmustfirstexistacrossabound-aryforthat‘amountandradius’tobeapplied.Begin adjusting the Adobe Photoshop Un-sharpMask(USM)filteratalowRadiussettingaround0.5to2.0pixelsand0ThresholdwithahighAmount(100%to500%.).

Unsharp Mask: 500% 1.0 pixels, 30 Levels

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