Nikon L31 has a
16.0MP 1/2.3' (6.17 x 4.55 mm ) sized CMOS sensor . On the other hand, Olympus 7040 has a
14.0MP 1/2.3-inch (6.08 x 4.56 mm ) sized CCD sensor and features TruePic III processor.
Below you can see the L31 and 7040 sensor size comparison.
Nikon L31 and Olympus 7040 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.