Canon ELPH 340 HS has a
16.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CMOS sensor and features DIGIC 4+ processor. On the other hand, 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.
Below you can see the ELPH 340 HS and SD940 IS sensor size comparison.
Canon ELPH 340 HS and Canon SD940 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. On the other hand, since Canon SD940 IS has 32% larger pixel area (2.34µm2 vs 1.76µm2) compared to Canon ELPH 340 HS, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.