Canon A1100 IS has a
12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor and features Digic 4 processor. On the other hand, Canon A800 has a
10.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor and features DIGIC 3 processor.
Below you can see the A1100 IS and A800 sensor size comparison.
Canon A1100 IS and Canon A800 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.