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