A Very Brief History of Focusing
Back in the days when ships were made of wood and photographers of iron, you would focus your lens by moving the whole lens (or the whole optical assembly) closer or further from the imaging surface. This worked just fine but it was slow and often a bit clumsy. Later on, lenses were designed so that just one element or groups of elements moved on a linear track with a tooth-and-rail gear. If you’ve ever used a macro rail or shot with a Lomography Petzval lens you’ve done this.Before long someone figured out that it would be convenient if you moved the focusing element nearer or further by just turning a barrel on your lens instead of twirling a knob. This is where the helicoid barrels of which we speak so often came into play. (Helicoids also move zoom elements in zoom lenses, but zoom lenses didn’t come along for many years.) The moving lens element(s) have rollers that pass through the slots in the helicoid barrel. When the barrel is rotated the groove moves the lens forward or backward within the barrel, rotating it as they do.Much later on, autofocus was developed. The ‘auto’ part of that required that the lens has a motor to move the focusing element without any input from the photographer. At first, small, geared direct current (DC) motors were used for this purpose.In the 1990s, ring USM (ultrasonic motors), which were definitely quieter, faster, and probably more accurate than DC motors, became commonly used.There have been some other motors used for autofocus. Stepper motors, a type of brushless electrical motor that moves more accurately than regular motors, are used in some lenses for improved focus accuracy (Canon’s STM motors are an example). Tamron’s Piezo Drive uses a non-ring shaped USM to move the focus. It is smaller and lighter than a ring USM, but I’m uncertain if it was better or worse as far as accuracy and speed.The terminology gets a little confusing here because a ring USM motor is technically a type of Piezo-electric motor. The important thing is that for all of these motors the focusing element moved in a helicoid slot, moving forwards and backwards as the slotted barrel rotated.To get details visit our site
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