Olympus SP-620 UZ has a
16.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor and features TruePic III+ processor. On the other hand, Olympus SP-820UZ has a
14.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CMOS sensor .
Below you can see the SP-620 UZ and SP-820UZ sensor size comparison.
Olympus SP-620 UZ and Olympus SP-820UZ 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 SP-820UZ has 15% larger pixel area (2.04µm2 vs 1.76µm2) compared to Olympus SP-620 UZ, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.