Us­ing aut­o­fo­cus mo­tors

NPhoto - - FEATURE -

The longer the fo­cal length of the lens, the shal­lower the depth of field, and as tele­photo zooms are mostly used to shoot fast-mov­ing sub­jects, it’s im­por­tant that aut­o­fo­cus is ag­ile. AF mo­tors come in two main types: mi­cro-mo­tor, which uses an elec­tric mo­tor to move the fo­cus­ing el­e­ments within the lens and are usu­ally cheaper but rel­a­tively slow and noisy; and ul­tra­sonic, which use piezo­elec­tric pulses to drive the el­e­ments ul­tra­son­i­cally, and are quicker and qui­eter. A ‘ring-type’ ul­tra­sonic vari­ant uses large rings that en­cir­cle the di­am­e­ter of the lens, and are found in the fastest-fo­cus­ing lenses.

Im­age: Getty

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