[Rover]

Also, this lens has a similar/same double gauss optical design as the above mentioned cheap Canon EF 50mm f1.8/EF 50mm f1.8 II and its follow up, the EF 50mm f1.8 STM. That means all elements move during focussing, so maybe that explains the choice for DC motor. The EF 50mm STM costs less than half than this Sony, though.

To be fair, it does add an aspherical element though. No idea if there's a big effect on image quality, however.