FujiFilm S2800HD has a 14.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor . On the other hand, Olympus SZ-31MR iHS has a 16.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features Dual TruePic V processor.
Below you can see the S2800HD and SZ-31MR iHS sensor size comparison.
FujiFilm S2800HD and Olympus SZ-31MR iHS have 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 FujiFilm S2800HD has 15% larger pixel area compared to Olympus SZ-31MR iHS, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.