Panasonic ZS5 has a
12.0MP 1/2.3-inch (6.08 x 4.56 mm ) sized CCD sensor and features Venus Engine HD II processor. On the other hand, Panasonic ZS8 has a
14.0MP 1/2.3-inch (6.08 x 4.56 mm ) sized CCD sensor and features Venus Engine FHD processor.
Below you can see the ZS5 and ZS8 sensor size comparison.
Panasonic ZS5 and Panasonic ZS8 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 Panasonic ZS5 has 16% larger pixel area compared to Panasonic ZS8, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.