Canon SD780 IS has a
12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor . On the other hand, Canon Elph 115 IS has a
16.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features DIGIC 5 processor.
Below you can see the SD780 IS and Elph 115 IS sensor size comparison.
Canon SD780 IS and Canon Elph 115 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 SD780 IS has 32% larger pixel area compared to Canon Elph 115 IS, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.