Both Fujifilm S8100fd and Olympus E-420 have 10.0 MP resolution sensors but Fujifilm S8100fd's sensor is 1/2.3-inch (6.17 x 4.55 mm ) and Olympus E-420's sensor is Four Thirds (17.3 x 13 mm ).
Since Olympus E-420's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Fujifilm S8100fd.
Below you can see the S8100fd and E-420 sensor size comparison.
As seen above, Olympus E-420 has a 8.0x Larger sensor area than Fujifilm S8100fd. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.