How aut­o­fo­cus works

Digital Camera World - - CAMERA COLLEGE -

Cam­era man­u­fac­tur­ers might use dif­fer­ent brand names for their AF sys­tems, but they are largely based on two dis­tinct meth­ods for driv­ing the aut­o­fo­cus: phase de­tec­tion and con­trast de­tec­tion. Some cam­eras use a com­bi­na­tion of the two for ‘hy­brid’ aut­o­fo­cus when shoot­ing in Live View.

Phase de­tec­tion AF uses dis­tance in­for­ma­tion to de­ter­mine whether a sub­ject cov­ered by the AF points is sharply fo­cused. SLRs use a ded­i­cated aut­o­fo­cus mod­ule for phase de­tec­tion. As a re­sult, it’s a rapid-re­sponse sys­tem, although it can be prone to er­rors, as it re­lies on the lens, the AF mod­ule and the AF points work­ing in har­mony. This is not al­ways the case, which is why some cam­eras in­clude an aut­o­fo­cus cal­i­bra­tion op­tion. Con­trast de­tec­tion isn’t af­fected by fo­cus­ing er­rors in the same way: it’s the fi­nal pic­ture cap­tured on the imag­ing sen­sor that’s used to set the fo­cus. You also have more free­dom for po­si­tion­ing the point of fo­cus, as you’re not re­ly­ing on fixed phase-de­tec­tion AF points.

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