Ricoh GXR GR Lens A12 28mm has a
12.0MP APS-C (23.6 x 15.7 mm ) sized CMOS sensor and features GR Engine III processor. On the other hand, Nikon D5100 has a
16.0MP APS-C (23.6 x 15.7 mm ) sized CMOS sensor and features Expeed 2 processor.
Below you can see the GXR GR Lens A12 28mm F2.5 and D5100 sensor size comparison.
Ricoh GXR GR Lens A12 28mm and Nikon D5100 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 GXR GR Lens A12 28mm has 31% larger pixel area compared to Nikon D5100, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.