Nikon S640 has a
12.0MP 1/2.3-inch (6.08 x 4.56 mm ) sized CCD sensor and features Expeed processor. On the other hand, Fujifilm S8100fd has a
10.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor .
Below you can see the S640 and S8100fd sensor size comparison.
Nikon S640 and Fujifilm S8100fd have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.