Panasonic GF8 has a
16.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features Venus Engine processor. On the other hand, Olympus E-P1 has a
12.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features TruePic V processor.
Below you can see the GF8 and E-P1 sensor size comparison.
Panasonic GF8 and Olympus E-P1 have the same sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Olympus E-P1 has 29% larger pixel area (18.45µm2 vs 14.21µm2) compared to Panasonic GF8, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.