Even between de DA* lenses you see very different preformance depending on the lens so you can not say anything with only one model DC lens when it comes to accuracy, speed and even reliablity, the DA* prime lenses for example and the DA*60-250 looks relative quite problem free.
Besides that the DA* lenses have bigger elements and the lens are tighter shield, so those SDm motors need to move more weight with more firction then the DA18-135 lens so is it a fair comparison?
About the technology.
DC is a burshless direct current electro motor, so it's like a regular elctro motor that you probably know.
SDM is different, it's a ultra/super sonic motor, it has some kind of feets and they vibrate in a certain way and they more or less walk actually.
I believe that when you tighten the screw on the SDM you actually bring those feets closer to their contact but this is speculating on my side though.
As for the elements with the DA*16-50 and 50-135 the front elements focus and the back ones are used for zooming.
As to what fails with SDM, i've no idea but since it only effect the zooms i'm actually inclined to say it might be design flaw in the AF mechanisme/gears rather then the SDM themselves. But SDM use friction to move the focus so they will wear out over time but this doesn't explain the failures.
Last edited by Anvh; 02-09-2012 at 02:14 PM.