Ricoh GR Digital IV has a
10.0MP 1/1.7-inch (7.44 x 5.58 mm ) sized CCD sensor . On the other hand, Olympus XZ-2 iHS has a
12.0MP 1/1.7-inch (7.44 x 5.58 mm ) sized CMOS sensor .
Below you can see the GR Digital IV and XZ-2 iHS sensor size comparison.
Ricoh GR Digital IV and Olympus XZ-2 iHS 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 Ricoh GR Digital IV has 18% larger pixel area compared to Olympus XZ-2 iHS, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.