The stepper motor returns
Stepper motors (STM) have been around decades longer than ultrasonic, and have been widely used for controlling electronic components for some time. By moving in steps (regulated amounts), stepper motors deliver the exact control required for applications such as; driving disk drives computer printers and, scanners. Unlike a basic DC motor, the stepper motor needs no sensor or encoder to provide feedback on the distance the motor has moved. Specific control is encoded into the motor design. conversely, early stepper motors contained a couple of paramount drawbacks. First was their extreme noise, which occurred as the current driving the motor switched on and off using a step function while the motor lunged forward to the subsequent step, creating considerable, audible vibrations. The second issue was that if they were manually manipulated or back driven their position memory could be lost, which in turn made an encoder exactly like the one required for simpler DC motors necessary.
As encoders became less expensive and smaller, stepper motors were frequently supplanted by quieter, simpler, cheaper DC motors joined with encoders for determining their location. Then ultrasonic motors were introduced, needing a tiny proportion of the power while generating a small amount of the noise DC motors created, replacing both stepper and DC motors in many noise and performance sensitive applications. Meanwhile, stepper motor engineers realized they could apply a current which varied to their motors to allow them to function a great deal smoother, and therefore quieter. These new micro-stepping motors can now be moved even more precisely while much quieter than previous versions.
Canon again innovates, this time using quiet stepper motors
Canon leaves AF video critics speechless with their new lenses featuring ultra-quiet stepper motors. Now, after twenty years since it pioneered ultrasonic motors in lenses, Canon is again the forefront of SLR photography by returning stepper motor engineering with its new STM lens family. The main idea is to essentially silent focusing, necessary while recording video. In addition they focus quicker than the low-end Canon DC motor-powered lenses. However STM lenses aren't for everyone. Canon concedes that their STM motors are not as fast as their USM high-end lenses, at least at this point. They are also cost more than basic DC-motor-driven editions. A big advantage of USM lenses along with other lens designs such as the Nikon AF-S is the photographer's capability to manually override the autofocus by simply turning the focusing ring, without changing modes. Canon has been able to duplicate this ability in its new 18-135mm and 40mm STM lenses.
Time will tell us if combining the Rebel T4i and STM lenses, along with similar arrangements from other manufactures will mean the end of dedicated camcorders at last, however the latest micro-stepper motor technology will certainly make it more probable, just as 25 years ago when Canon’s ultrasonic motor innovation helped hasten the rangefinder camera decline .
Contrast Detect Autofocus
Contrast Detect Autofocus (CDAF) is used when no SLR mirror is at hand, which would embrace LiveView on any SLR camera (making use of a rear LCD screen), or with an Electronic Viewfinder (such as the Micro Four-Thirds camera system).
Phase Detect Autofocus
Phase Detect Autofocus (PDAF) is employed on an SLR containing a mirror, framing through the camera's viewfinder. Phase Detect needs a separate sensor underneath the mirror, whereas CDAF gets its data from actual imaging sensor.
However, Canon is not the first manufacturer to use a micro-stepping motor to enable quiet focus, only the first manufacturer to employ them on a single lens reflex. For instance, Sigma, has announced line of what they call Digital Neo (DN) lenses which employ linear motors which work on mirrorless cameras and provide similar silent operation and focusing accuracy to the Canon STM versions. The new Sigma DN 19mm f/2.8 has ultra fast focus (faster than most Olympus or Panasonic lens). The Sigma 19mm DN hunt at all. (I would suppose it uses discrete step), is completely silent, although it needs one second initialization (add one second blackout to the camera power-up) and confirms the stepper motor requires indexing at the start of each cycle
The Sigma linear motor appears to be the same as the Canon STM. I believe we are seeing the future in the STM (stepper motor) as older version PDAF lens are terribly slow to focus using CDAF live view. older PDAF technology (USm or arc motor) is unsuitable for CDAF, they will work, but are extremely slow and labor terribly. You can't capture fast continuous autofocus video using PDAF lenses although you can capture fast continuous autofocus video using a CDAF lens
The Tamron stepping motor actuator provides finely tuned angular rotation control, and it directly powers the focusing mechanism without needing intermediate reduction gears, it also delivers superbly quiet performance.
Other mirrorless cameras lens makers also have caught onto micro-stepper motor advantages. In addition to their nearly stealth noise, micro-steppers provide very exacting incremental positioning. This is enormously helpful in speeding up contrast-detection autofocus required for mirrorless system cameras (phase detection autofocus, which is used to acquire still images in most DSLR cameras, is more intelligent about knowing the direction in which to position the lens, while therefore does not depend on fast, small movements for fine focus).
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