Canon SX170 IS has a
16.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor and features Digic 4 processor. On the other hand, Canon SX40 HS has a
12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized BSI-CMOS sensor .
Below you can see the SX170 IS and SX40 HS sensor size comparison.
Canon SX170 IS and Canon SX40 HS 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 SX40 HS has 32% larger pixel area (2.34µm2 vs 1.76µm2) compared to Canon SX170 IS, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.