Astigmatism and Lateral Color: Why Performance is Lens + Sensor (aka why do my expensive lenses suck on mirrorless)
Added to Making Sharp Images is a discussion of why even world-class lenses can show poor performance on mirrorless cameras (in particular).
Optical performance on digital can vary considerably depending on sensor design. This piece should be an eye-opener for anyone looking to understand lens behavior on digital.
This double image below is not motion blur. Rather, it is a direct result of the way some lens designs interact with a digital sensor.
Martin D writes:
DIGLLOYD: Many factors in digital. I expect some exciting things in lenses and sensors starting this fall.