Although taken for granted these days, autofocus (AF) used to be a premium feature on film- and digital cameras, with [Markus Kohlpaintner] taking us through an exhaustive overview of Canon’s AF systems and how they work. On Canon cameras AF became a standard feature with the introduction of its EF lenses in 1987, which are found on its EOS SLR (single-lens reflex) series of professional and prosumer cameras.
Over the decades, Canon has used many different AF drive mechanisms within these lenses, all with their own pros and cons. The article goes through each of them, starting with the original Arc-Form Drive (AFD) and ending with the newest Voice Coil Motor (VCM), showing their internal construction. Of note are the USM (ultrasonic motor) types of AF systems that use a piezoelectric motor, the functioning of which using a traveling wave across the stator is also detailed, including the integrated feedback control system.
Ultimately the end user is mostly concerned with how well the AF works, of course. Here the biggest difference is probably whether manual adjustment is possible, with not all AF systems supporting full-time manual adjustment. With the newer AF systems this manual adjustment is now performed digitally rather than with a direct coupling. Although few people probably give AF much thought, it’s fascinating to see how much engineering went into these complex systems before even touching upon the algorithms that decide what to focus on in a scene.
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