Doesn'tthis woman know how to hold a camera?Of course she does,but the AF didn't work when the camera washorizontal, so she's giving it a diagonaltwist (see text as to why).Thereare three grades of AF SLR systems based on autofocussensor complexity: A) Simple single central AFsensor (the viewfinder's AF area is outlined bya smallish rectangle or brackets). B) More sophisticatedmultiple sensors covering an appreciably largecentral area of the picture (with multiple finderrectangles or an enlarged central sensitive rectangleor brackets in the viewfinder). C) The new gee-whiztop-of-the-line multisensors that can switch fromsensor to sensor at your command or to followsubject activity.Ifyour camera has a single central sensor, makesure you put the central viewfinder AF rectangledirectly over the subject you wish sharply focused.If you don't want the subject in the centerof the picture, focus the subject centrallynevertheless, but then hold focus using yourcamera's autofocus lock, recompose the picture,and then press the shutter release.
Manysingle central sensors are sensitive only tothe vertical lines in a subject: no verticallines, no autofocus. If your camera refusesto autofocus on a subject, turn your camerato a 45-degree angle (as shown in the photoabove) and try autofocusing again. If it nowfocuses, hold focus with the AF lock, recompose,and only then press the shutter release.
Multipleautofocus sensors are quite effective in coveringa large area. However, sometimes they don'tknow just what subjects within the area youdo want focused sharply. Some sensors are programmedto focus on the nearest sharply defined image.So a pronounced foreground area may be focusedsharply instead of your slightly more distant,less defined central subject. Wrong-subjectautofocus can also occur with complex groupingsof people at a party where individuals are atvarying distances from the camera. The cameradoesn't know where to focus. Only you do. Eithershift to central AF and lock focus on the subjectthat's important for you or manually selecta specific sensor that covers the importantsubject area.
Withautoswitching multisensor AF cameras, the mostimportant advice we can give you is to read theinstruction book carefully and then try out everyAF feature yourself. These cameras can yield thehighest percentage of successfully focused picturesever, but they do not have brains. Blind faithin multisensor systems is not enough. Use yourhead.Nowfor some random AF thoughts: Don't grump ifyour camera refuses to autofocus on near impossibletest subjects such as blank walls or venetianblinds. AF sensors hate multiple-lined subjects—likeblinds. Stick to typical subjects you wouldshoot. Other subjects where AF may balk: low-contrastobjects, infinity horizon lines, shots throughglass having reflections.
MostAF sensors won't operate well or at all withlenses having smaller than f/5.6 maximum apertures.
When shooting action with an AF camera, switchfrom single focus to continuous focus—if yourcamera allows the switch.
WhenAF is balky or you have doubts, don't miss thepicture attempting to get it to autofocus. Switchto manual focus immediately!
