Digital SLR Photography

AUTOFOCUS SySTemSGettinG fullyto Gripswithyour camera's sophisticatedautofocussystemwillensurethe imaGesthatyoutakeareas sharpas possibleThe technology built into evenentry-level DSlRs these days isphenomenal. All that computerwizardry processing information inmilliseconds to give us high quality imagesatthe touchof a button. the trouble is, becausetoday’s digital cameras are so good, we tendto rely on them a bit too much – we expectperfectresults every time and when we don’tget what we want, we blame the camera!this is especially true when it comes tofocusing.Weassume that autofocusing is sogood now that pin-sharp images areguaranteed. Moreoften than not they are,but you do need to understand the basicsofhow your camera’s AF system works, whichsettings to use and when, if you want toachieve the bestresults every time.``````AFPOINTS : mULTIPOINTAFAll cameras use a multi-point AF system and,while the numberof points varies frommodel to model. they all work in a similarway, in that you can choose to make all thepoints activeat once, or select specificgroupsof points, or select just one. howevermany orfew you choose, when you partiallypress the shutter button, the points that arefunctioning and determining what the lensfocuses on usually flashred. Preferencetends to be given to whatever is closest tothe camera. if you’re an absolute beginner,multi-point AF can be handy as it gives youone less thing to think about. if you’re takinggeneral shots with your lens set to a smallishaperture – f/8, f/11, f/16,for example – multi-point AF can also work fine because youwant everything in sharpfocus and no singleelement in the scene needs to be givenpriority. however, because you have littlecontrol over what the lensfocuses on, insituations where you need tofocus onsomething specific, and even more so whenyou’re shootingat a wide lens aperture sodepth-of-field is limited, it’s best not to haveall the AF points active asfocus error is likely.the exception is with DSlRs where you canselect a mainfocus point then nominateothers around it asfocus‘assist’ points so thatif your subject strays beyond the mainfocuspoint the others willkeep it infocus. ifamoving subject travels across the frame, andremainsat roughly the same distance fromthe camera, multi-point AF can also work``````THe mAINAUTOFOCUS SySTemSthere are two main typesof autofocussystem: phase detection and contrastdetection.Phase detection is the defaultfocusing systemfor DSlRs. here’s how itworks: when light enters through the lens,mostof it is bounced upoffthereflex mirrorinto the viewfinder so you can see whatyou’re shooting, but someof that light is alsoreflected down onto a sub-mirror, whichthen bounces it onto the autofocus (AF)sensor. on topof the AF sensor is a beamsplitter that divides the light into two beams.the sensor analyses these beams todetermine how far out-of-focus the image isand whetherfocus is in frontof, or behindthefocal plane, then adjustsfocusaccordingly. the numberof sensors in DSlRsvaries from model to model, but generally,the more expensive the camera, the moresensors or ‘points’ there are. A budget modelmay have nine points and a pro model 150 ormore.you can either activate all the points,then leave the camera to decide which touse, or select a specific point or clusterformore control. Phase detection AF is superbbut it’s not totallyfoolproof and even the bestsystems can struggle when used in low lightor if you try tofocus on a plain or non-textured area such as blue sky or a white wall.contrast detection works on the principlethat the contrast in an image isat its highestwhen that image is sharplyfocused, so theAF system adjustsfocus and analysescontrast,often going past the critical pointso it knows where it is then adjusting backand lockingfocus. Mirrorless cameras usecontrast detectionat all times, whereasDSlRs use it when in liveView. contrastdetection canfocus on any partof the imagearea, which is why some cameras havefeatures such asFace Detection. however,it’s usually slower than phase detection, andnot as good with moving subjects.Some DSlRsoffer ‘hybrid AF’ in liveViewand Movie modes, which uses both PhaseDetection and contrast Detection AF toimprovefocus accuracy and speed.68 Digital slr PhotographyNove mber2016TheBeginner’sGuideit all the time and it becomes second nature.the second method is to select a singleoff-centre AF point and use that. if you’retaking lotsof shotsof anoff-centre subject,especially if that subject is moving, thisworks better than using the central AF pointas you don’t need tore-composeafterlockingfocus. it’s also preferable when usinga wide aperture that gives a shallow depth-of-field – lockingfocus with the central AFpoint thenre-composing may mean thatthe point youfocused on is no longer infocus,resulting in an unsharp subject.``````because as it falls behind another AF pointthat point will become active andkeep thesubject in sharpfocus.Serious photographers tend to use singlepoint AF rather than multi-point, so as tochoose exactly what the lensfocuses on.there are two ways to do this. the first is toselect the central AF point as it’s the mostsensitive point your camera has. if the areayou want tofocus on isoff-centre, place thecentral AF point over that area, half-pressthe shutterrelease to lockfocus,re-compose and fire. this sounds fiddly, but do``````Ab ove:single-pointafrequires accuracywhenshooting using a shallowdepth-of-field.Left:multi-pointafcan beusedto help track movingobjects across the frame,but can also becomeconfusedwhen multiplesubjects are present.``````ben hAll``````joRDAn but teRS