FujiFilm S2800HD has a 14.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor . On the other hand, Fujifilm S8600 has a 16.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor .
Below you can see the S2800HD and S8600 sensor size comparison.
FujiFilm S2800HD and Fujifilm S8600 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 Fujifilm S8600, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.