Olympus SZ-12 has a
14.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor . On the other hand, Olympus SZ-15 has a
16.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor .
Below you can see the SZ-12 and SZ-15 sensor size comparison.
Olympus SZ-12 and Olympus SZ-15 have the same 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 Olympus SZ-12 has 15% larger pixel area compared to Olympus SZ-15, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.