If you put a thin linear obstruction in front of the telescope, then star images will contain a single diffraction spike when the image is in focus. When out of focus, there will be multiple spikes. You just adjust the focus until there's one spike. This means that star images are about 2 pixels wide. With less resolution, the image looks blocky.
More resolution is wasted.
Bill says this factor is often overlooked when people buy a CCD camera. Exposure time is related to polar alignment. Bill takes about 20 minutes using the drift method to align his scope. This give alignment that's good enough for a 2 minute exposure. Finally, because the images are digital, you can do all sorts of tricks to filter out light pollution, camera noise, etc.