Casio EX-Z450 has a 12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor . On the other hand, Ricoh PX has a 16.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor and features Smooth Imaging Engine IV processor.
Below you can see the EX-Z450 and PX sensor size comparison.
Casio EX-Z450 and Ricoh PX 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-Z450 has 18% larger pixel area compared to Ricoh PX, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.