zz from dpreview
STM lenses are not new mounts nor are they intend to supplant existing EF/EF
-s lenses. The electronics of the EF is still the same, and the latest in
optical design and IS and coatings are still employed. Only the motor
changed.
STM lenses are to complement existing EF and EF-s lenses. They are additions
to the Canon lens lineup to address those who do lots of video.
STM lenses are more specific to video shooting because:
1. They don't jerk/snap focus abruptly in video mode as with other motors
2. They AF smoother in achieving focus so that is not jarring to the shoot
3. There is also a less tendency to overshoot the subject of AF and hunt
noticeably before focus is achieved
4. They are supposed to be quiet too, so that you will pick up less noise as
you focus with STM.
STM lenses tend to achieve focus better in video mode in that they work well
in fine increment adjustments. They are also less taxing to the motors
compared to other designs that may actually prolong your lens motors life if
you shoot a lot of video and use AF in the process.
USM motors will still be faster than STM and more advantages for pure stills
shooting.
