Quote Originally Posted by Sushigaijin
True, but APS pixel density is higher than 4/3 - the 50d's 15 mp is higher than the density of it's closest 4/3 competition, the 10mp E3.
No it's not. The Canon has 15.5 million actual pixels (15.1 million effective), on a sensor that is 329 mm square, for a total density of 47.1K pixels per square mm.

The Olympus has 11.8 million actual pixels (with 10.1 million effective) on a sensor that is 225 mm square, for a total density of 52.4K pixels per square mm.

Incidentally, the reason Olympus has so many more actual pixels vs. effective pixels relates to their live view implementation, which basically consumes around 1/8th of the total sensor resolution.

That being said, those two density numbers are pretty darn close, so there isn't any fundamental reason that a 16 MP Canon APS-C sensor would be any more sensitive than a 10 MP 4/3 sensor of the same generation. Other than the fact that Canon has been making their own sensors for quite some time, and has lots of experience at high ISO sensitivity. Of course, the 50d's sensor is actually a generation newer than that of the E-3. The more direct competitor from a silicon generation standpoint is the E-30's sensor.

In any case, BOTH an APS-C sensor and a 4/3 sensor get the benefit of the sweet-spot effect when only using the center portion of a lens that's designed for the full 35mm frame. Most good lenses have no problem resolving that density at the center, at least when stopped down 1-2 stops from the max, but the further away one gets from the center point, the more it becomes an issue.