Nikon L31 has a 16.0MP 1/2.3' (6.17 x 4.55 mm ) sized CMOS sensor . On the other hand, Casio EX-ZS5 has a 14.0MP 1/2.3-inch (6.16 x 4.62 mm ) sized CCD sensor and features Exilim Engine 5.0 processor.
Below you can see the L31 and EX-ZS5 sensor size comparison.
Sensor Size and Resolution Comparison image of Nikon L31 and Casio EX-ZS5 Cameras
Nikon L31 and Casio EX-ZS5 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.