Canon SX510 HS has a
12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CMOS sensor and features Digic 4 processor. On the other hand, Canon SX400 IS has a
16.0MP 1/2.3 (6.17 x 4.55 mm ) sized CCD sensor and features Digic 4+ processor.
Below you can see the SX510 HS and SX400 IS sensor size comparison.
Canon SX510 HS and Canon SX400 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.