Casio EX-Z550 has a
14.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor . On the other hand, Canon SD780 IS has a
12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor .
Below you can see the EX-Z550 and SD780 IS sensor size comparison.
Casio EX-Z550 and Canon SD780 IS 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.