In order to build an accurate representation, the digitizer needs many more samples than it does for a low frequency area. After determining the area of highest spatial frequency, the digitizer calculates a sampling rate for the entire image. That speed is double the rate of the image's highest spatial frequency. In this way the digitizer captures all of the scene's subtle tonal nuance. Of course, the camera's sampling rate is not infinite, especially in lower priced models.
Pixel density depends on the amount of capacitors on the CCD chip.
This varies quite a bit between different makes and models of cameras. Generally, cameras are assigned spatial frequency rates that cover most situations photographers are likely to encounter. The apparent brightness of an object in the real world is quite different from its representation in a picture. Anyone who has ever gazed at the sun instinctively knows the difference between the actual object and a photograph of it. This may seem an academic distinction, but it is a key concept in digital imaging.