What is the difference and function between grid test and image quality of full-size Canon camera?

I think you can understand it this way.

Quan Huafu means that the CCD/CMOS area of the photosensitive element of a digital camera is as large as the film of the same specification (i.e. 36×24mm). However, due to the difficulty and high price of CCD/CMOS, most digital cameras are not made in Quan Huafu at present. CCD/CMOS is smaller than a 135 film, and its imaging area is also smaller. Generally speaking, DSLR is larger than small DC. The most common frame between them is APS-C frame. APS is called advanced photo system and has been eliminated. There are three kinds of APS films. ...