Aren't all thoses "digital" lens only optimized for the 1.x factor?
Sometimes yes somethimes no. Digital sensors need for the light to hit the sensor in a more or less orthagonal way. Without this at the edges of a camera such as the 1ds will have severe vignetting. Then there are lenses such as the ef-s lens and the 4/3 system, which are only useable with smaller sensors.