Amateur Photographer - UK 2019-07-12)

subscribe 0330 333 1113I http://www.amateurphotographer.co.ukI 6 July 2019 19

AFmysteries

explained

1

Whatisthefocus

limiterswitchfor?

If yoursubjectis goingtobewithina

specific rangeit canbehelpfultousethe

lensfocuslimitertopreventit from hunting

acrosstheentirefocusrange.

2

Whatisfocusstacking?

Thistechniqueinvolvesshooting

a seriesof images,eachwitha slightly

differentfocusdistance.Theimagesare

thencompositedintoonethathasmuch

greaterdepthof fieldthananindividual

shot.It’sa verypopulartechnique with

macrophotographers.

3

WhydoesAFdependon

thelensI amusing?

Differentlenseshavedifferentmotorsthat

drivethefocusinganddifferentweight

elements.Also,lenseswitha wider

maximumapertureletin more light,

whichhelpstheAFsystem.

4

Whyarefull-frameDSLRs’

AFpointsnearthecentre?

Vignettingaswellasthemainand

secondarymirrorarrangementof a DSLR

meanthattheAFsensoris smallerthanthe

imagingsensor. As a result, the AF points

arecentral.

5

Canf/5.6-sensitiveAF

pointsbeusedatsmaller

apertures?

Yes.Thefocusingtakesplacewiththe

aperturewideopen,soaslongasyouuse

a lenswitha maximum aperture of f/5.6 or

greater, it will work.

Somecamerashavea seriesof customisation
settingsfortheAFsystem.Theseaffectaspects
suchashowtheAFrespondstoa changein
thesubjectdistanceorif thesubjectmoves
fromtheselectedAFpoint.
Manyphotographersassumethatit’sbestto
setthecameratoreactquicklytoanychange,
buta littlepausecanbea goodthing.For
example,if you’rephotographinga swimmer
doing the breaststroke towards you, with a fast

settingthecamerawillrefocusonthefarend

of thepooleverytimetheybobunderwater.

Thenwhentheyreappearit willrefocusagain,

makinga majoradjustment.A slowerreaction

willhelpthecamerastayonthesubject.

Togetthebestfromthesecustomisation

systemsyouneedtoconsiderwhatyou’re

photographingaswellastheshooting

conditions– forexample,whetheranyobjects

are likely to come between you and the subject.

Focusingsystemsneedlightandcontrastto
work.It’sthelackof contrastthatcanleavea
camerastrugglingtofocusona well-litsheet
of paperora smoothwall.If youcanfindsome
pointof contrast,evenif it’sjusta joinin the
wallpaper,it willoftensnapintofocusquickly.
Becausethephase-detectionsensorsin a
DSLRareoftenarrangedin lines,thedetail
needstocrossa lineratherthanrunalongit for
it tobe‘visible’totheAFsystem.Consequently,
it’s a good idea to understand your camera’s

AFsensorarrangement.It’salsowhycross-

typeAFpointsareuseful– theycandetect

detailsin morethanonedirection.

If yourcamerais strugglingtofocus,lookfor

anareaof contrastorconsiderswitchingtothe

centralAFpoint.Aswellasbeingcross-type,

thisis usuallythemostsensitivein a DSLR.

Camerassometimeshavemultiplecross-

typeAFpointsandit’sworthknowingwhere

theseare,especially when shooting action

in low light.

Knowinghowyour

subjectmovescanhelp

youdecideontheideal

AF customisation settings

Mostcamerasstruggletofocusona
silhouetteunlessyouputtheactiveAFpoint
over the edge where there’s some contrast

My AF system is struggling to lock on – what should I do?

Is it worth customising my AF system?