Amateur Photographer - UK (2019-08-30)

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Tech Talk

ProfessorNewmanon...

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

Perspective

O

neof the‘facts’

commonlypresented

in onlinephotography

tutorialsis thatthe

perspectiveof a photois affected

onlybythedistancebetween

cameraandsubject.I put‘fact’in

quotesbecause,in myopinion,this

is nota factbutanunhelpfulpiece

of adviceformostphotographers.

It’snotthattheessenceof the

statementis wrong,it’sjustthatit

doesn’tdescribeperspective.The

factbeingreferredto is this:the

relativeapparentsizeof objectsat

differentdistancesdependsonly

onthosedistances,andnoton

thefocallengthof a lens.Thisis

simplegeometry,astheapparent

sizeis determinedbytheangle

subtendedbytheobjectat the

lens,whichis givenbythearc

tangentof theobject’sphysical

sizedividedbyitsdistance.

Thiscanbeseenin theimages

above.Thesizerelationship

betweenthecottageandthegap

in thetreesis thesamein both

images.Thisis becausebothwere

takenfromthesameposition,so

thedistancefromthecamerato

treesandcameratocottageis the

samein eachcase.Hence,the

outcomeis thattheangle

subtendedat thelensforthetwo

objects is the same in each case.

So,is the‘perspectiveis thesame’

argumenttrueorfalse?I would

say‘false’forthesimplereason

thattherelativesizeof thetwo

objectsdoesnotdefinethe

‘perspective’asseenin theimage.

Thetwoimagesareverydifferent,

eventhoughtheyweretakenfrom

thesamepositionwiththecamera

pointingin thesamedirection.The

framingis whatdiffers,givenby

thedifferentangleof viewused.

Judgingperspective

The‘perspective’beingdiscussed

hereis whatanartistwouldcall

‘linearperspective’.Thisis a

geometrictrickwhichcreatesan

illusionof distancein animagethat

is actually2D.In a perspective

image,linesthatareparallelin the

depictedsubjectconvergetowards

a vanishingpointonthehorizon.

Theillusionof perspectiveworks

becausetheshrinkageof distant

objectsis oneof thecuesthatwe

useto determinethedepthof the

scenearoundus.It worksbased

ona priorassumptionsof the

sizeof commonobjects.Soin this

example,weknowapproximately

howlargea cottageshouldbe,and

thenuseitsviewedsizeasonecue

to determineitsdistance,andthe

relativesizeof otherobjectsto

determine their distance with

respectto us.Thehumanvisual

systemusesa numberof meansto

judgedepth.Themostimportantis

occlusion(i.e.anobjectin frontof

anotherobscuresit),followedby

shadowing(occlusionof light),

lighting,perspective,texture,depth

of field,stereopsisandmotion.

A photographercanmanipulate

apparentdepthbycreating

abnormalrelationshipsbetween

thesecues.In general,theworking

relationshipassumesarounda 40°

angleof view(theangleusedfor

‘normal’lenses).Thewiderangle

imagein theexampleis around

thatangle.Theotheris much

narrower,butthevisionsystem

evaluatesthevariousdepthcues

asthoughit wasstill40°.Both

thecottageandgapin thetrees

appearmuchcloser,whichin turn

makesthemseemclosertogether

in depththanthesizerelationship

wouldindicateif theywerefurther

away.Thus,theimageperspective

appearsto becompressed.To

gainthiscompressedeffect,a

photographerneedsto manipulate

bothsubjectdistanceandangle

of view– whichis whythe

statement‘perspectivedepends

onlyonsubjectdistance’is

unhelpful.Perspectiveis a property

of theviewedimage,not the

original 3D subject.

Perspectiveissomethingthatcanbemanipulated

in a photograph. Prof Bob discusses how it works

Do these two images have the same perspective? Some people would insist that they do

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