AF points ex­plained

The amount of aut­o­fo­cus points varies be­tween cam­eras – but in short, the more you pay, the more you get

Digital Camera World - - CAMERA COLLEGE -

En­try-level SLRs have around 7-9 AF points, whereas a high-end CSCs like the Sony A7 Mark III squeezes in 693. Cam­eras that of­fer a densely pop­u­lated ar­ray of AF points across the viewfinder make it eas­ier to fo­cus on sub­jects that aren’t in the cen­tre of the pic­ture. A large num­ber of AF points also makes for smoother fo­cus tracking, as the cam­era can ‘hand off’ aut­o­fo­cus from one AF point to the next to more ac­cu­rately fol­low a sub­ject mov­ing across the pic­ture.

AF point ar­ray

AF points are clus­tered to­wards the cen­tre of an SLR viewfinder. An AF point ar­ray typ­i­cally con­tains stan­dard AF sen­sors that mea­sure sharp­ness in one plane and ‘cross-type’ AF sen­sors that are more pre­cise.

How they work

The AF sen­sor de­tects a line of con­trast that ‘breaks’ the line of the sen­sor – such as the hori­zon here. The AF sys­tem pushes the lens’ fo­cus­ing back and forth un­til the ac­tive sen­sor is able to find this high-con­trast point.

Types of AF points

Be­gin­ner cam­eras may have one cross-type point, whereas high-end mod­els have many.

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