The need to combine speed for still image AF with smooth response and drive for video recording has been a challenge for engineers, because in most situations, what constitutes good AF performance in a lens differs significantly for still image shooting and for video:
- For still images, users normally want fast AF speed that can start and stop almost instantly and accurately put the sharpest focus where the user wanted it;
- For video recording, users normally want smooth consistent AF speed without sudden starts and stops.
AF motors originally designed for superior AF performance during still image shooting — like ring-type Ultrasonic Motors — tended to have less than ideal smoothness when asked to perform video-oriented AF. Despite the fact that stepping motors are good enought to cope both with still images and video recording, the users now demand for an even higher performance during video recording.
In order to meet the new requirements, the engineers implemented a slightly different approach in developing of the new AF motor — using the same principles of ultrasonic vibration converted into movement, they developed a direct-drive system that is incredibly compact, easily placed within the body of a lens and has tremendous AF performance. AF speed during still-image shooting rivals the high-end lenses with powerful ring-type ultrasonic motors — it’s nearly instantaneous. It’s a clear step forward from what we have come to expect from the consumer lenses based on Micro USM motors or the recent stepping motors. Linear motor is also outstanding in video applications — AF operation has tremendous smoothness and is free from the start-stop focus movement. Another huge factor in video shooting is that the AF operation is nearly silent — very important if a microphone is attached to the camera and vital if using the camera’s built-in microphone. Linear motor matches the relative silence of previous stepping motor designs during video recording. Due to it’s combination of focus characteristics, linear motor is an interesting and compelling possibility for many photographers.
At the moment, however, linear motor is optimized for lenses with small and lightweight focusing groups. Lenses with large and heavy focusing elements are better served with more powerful ring-type ultrasonic motors.
The accuracy of autofocus does not depend on type of autofocus drive (micromotor, stepping, linear or ultrasonic motor) but depends on focusing method (contrast or phase detection AF), efficiency of AF algorithms, lighting conditions and the number of other factors.
List of lenses equipped with linear motor: