Originally posted by beholder3 There ARE DC Ring motors. Asahiman hinted at such a drive in the dFAs.
Until someone opens his lens we will not know for sure.
BTW there are incredibly slow ring USM drives out there as well.
Well, ring motors are by definition ‘DC’, since they use direct current, but no, there’s no such thing as a ring motor that employs a brushless DC motor... because then the ring motor wouldn’t be a motor at all. It could be a “supersonic” micromotor, like the ill-fated original SDM, or Canon’s Micro USM, but then it wouldn’t be a ring-type drive at all. Ring-type motors use vibrations in very high frequencies, that are translated into rotational motion, to move a focusing group. This works nothing at all like a brushless DC motor.
I’d like to see an “incredibly slow” ring-type drive. I don’t believe it exists. But there certainly are manufacturers that play fast and loose with their naming schemes (almost all of them, actually), and use confusing terminology to make buyers believe that they’re going to get the same performance from a standard zoom or prime than from an exotic telephoto prime, when in reality the former have puny AF motors that work ‘OK’ (and show little, if any, difference with Pentax lenses), and the latter use very powerful motors - and that’s where the difference is really felt.
Ultimately, the industry is moving away from both types of AF motors in preference of linear drives, which are nowadays relatively cheap to produce and have very quick startup, low energy consumption, no friction, and the ability to change direction really quickly. By using two, or even four linear motors, faster AF speeds can be achieved over even the best ring-type drives with competitive energy consumption and extremely precise control. Many Micro Four Thirds and Sony E setups have single-point AF speeds and precision that need to be experienced to be believed, all thanks to linear drives. Pentax have also used this type of motor in their PLM lens - it’d have been fantastic if they had used this technology sooner.