Canon SD940 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, Samsung WB850F has a
16.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized BSI-CMOS sensor .
Below you can see the SD940 IS and WB850F sensor size comparison.
Canon SD940 IS and Samsung WB850F 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 Canon SD940 IS has 32% larger pixel area compared to Samsung WB850F, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.