Digital SLR Photography

Nove mber2016Digital slr Photography 69An increasing numberofdigitalcameras now havetouchscreens, and someofthoseeven allowyou tochoosewhere thelensfocusesbytouching the screen inaspecific place –tofocuson a person, say. Thistechnology has been aroundin smart phonesfor a while so it’sonly logical thatcamera manufacturers are nowstartingtotake advantageofit too. Is it any good?For snapshots, probably, butfor seriousphotography wethink not.You havetobelookingat thecamera’s preview screentousetouch AF,andfew serious photographers do that. It’s alsoimpracticalfor moving subjects asbythe timeyou touch the screen and thecamerafocuses,your subject will havemoved.``````There are two typesofAF sensor used in DSLRs:cross-type and linear. Mostcameras use a mixtureofboth, with the cross-type being clustered around thecentral areaofthe frame and linear sensorstowardsthe edges. Cross-type sensorsconsistoftwo linearsensors crossing at 90°, hence the name. They’remore accurate than linear sensors – especially in lowlight – as they can detect bothvertical and horizontallight patterns whereas linear sensorscan only work inone dimension. Therefore, the more cross-typesyourcamera has, the more effectivefocusing will be.On some high-end DSLRs, withlots ofcross-typesensors, someofthose sensors only work ascross-type when used with ‘fast’lenses that haveamaximum apertureoff/2.8 or wider, so ifyou’re usingzooms with a maximum apertureoff/4 orf/5.6, someofthese cross-types willwork like linear sensors.You also needtobeaware that some DSLRscanonly autofocus if thelens has a maximum apertureof``````f/5.6 or wider while with others it’sf/8 or wider. Thisisn’t normally an issue, but ifyou fit ateleconverter,whichreduces the effective maximum apertureofthelens, itcan be.A 1.4xteleconverterreduces themaximum aperturebya stop, sof/4 becomesf/5.6andf/5.6 becomesf/8. A 2xteleconverterreduces itbytwo stops, sof/4 becomesf/8 andf/5.6 becomesf/11.If you use aconverter and AF doesn'twork, this is why.Af sensors Touch focus