>>3280857Because of how phase detect works - it calculates the distance to the subject, sends a message to the lens, and the lens executes the command - DSLR lenses are geared to snap into focus instantly.
Contrast detect works by making a series of calculations to send a series of messages to the lens, bringing the subject into focus gradually, little by little and with mediated feedback going back and forth between the sensor and lens. Even the mirrorless camera with the fastest AF implementations can only "gradually snap" the subject into focus compared to how a DSLR works.
Hence, lenses designed for mirrorless cameras almost all tend to use a stepping motor that's geared to facilitate rapid changes in speed, which DSLR lenses traditionally aren't.
