Quad Linear Motor of XF90mmF2 LM WR
XF90mmF2 R LM WR is the first prime XF lens that is driven by linear motor.
Linear motor is fast and quiet. And on top of that, it is accurate. It moves the lens unit directly to the focus position. There are no other unnecessary mechanism between the lens and the motor.
This sets the linear motor apart from other motors: Ultrasonic motor, DC motor, Stepping motor and etc., It brings the resolution peak to the focal point. This accuracy is very effective for the optical design of XF lens.
But linear motor has its disadvantage as well. The motor is not suitable for heavy large diameter lens due to lacks of torque. And this is one of the main reasons to why there never was a prime XF lens with linear motor.
High torque can be achieved with large linear motor unit with larger magnet and lots of coil wind up, but this would result in bulky lens size.
So what FUJIFILM did to get away with the problem was to utilize 4 magnets to achieve strong magnetic force = higher torque. (Normally 2 magnets are utilized).
And coil was wind up on the circumference of the lens. (Normally coil is wind up on each magnet.)
This idea led to the smaller sized lens.
The filter size of XF90mm is 62mm. Within this slim lens exists the large optical elements and powerful driving unit.