At first, video-capable DSLRs were widely viewed as a gimmick. Low resolution, manual focusing, lousy audio, and short clip lengths conspired to make the feature useful mostly for short 1-person narrations of scenes and quick how-to videos. Recent model DSLRs have blown through those objections, providing high quality HD video, and audio with external microphones. Autofocus has been one of the last barriers to fall. Full-time autofocus has finally been added in some recent high-end models, and now in the new entry-level Canon EOS Rebel T4i (650D).
Unfortunately, until now, DSLR autofocus motors have been audible when using the on-board microphones found in entry-level cameras — SLR lenses were never designed with complete silence in mind. Canon’s new STM (STepper Motor) lenses, coupled with the full-time Movie Servo mode on the Rebel T4i should finally quiet critics of DSLR video. Electronics buffs may be shaking their head about the fuss over stepper motors, a venerable technology that is being reborn for these new lenses. There is an interesting story behind how camera autofocus drive mechanisms evolved, and we’ll see that Canon has been at the forefront of much of it.