Nikon L31 has a 16.0MP 1/2.3' (6.17 x 4.55 mm ) sized CMOS sensor . On the other hand, Casio EX-Z550 has a 14.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor .
Below you can see the L31 and EX-Z550 sensor size comparison.
Nikon L31 and Casio EX-Z550 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 Casio EX-Z550 has 14% larger pixel area (2.01µm2 vs 1.76µm2) compared to Nikon L31, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.