Ricoh GR Digital III has a
10.0MP 1/1.7-inch (7.44 x 5.58 mm ) sized CCD sensor and features GR engine III processor. On the other hand, Nikon P7800 has a
12.0MP 1/1.7-inch (7.44 x 5.58 mm ) sized BSI-CMOS sensor .
Below you can see the GR Digital III and P7800 sensor size comparison.
Ricoh GR Digital III and Nikon P7800 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 III has 20% larger pixel area compared to Nikon P7800, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.