This assumes, of course, that light falling directly on a virtual pixel will actually spread into the four real pixels from which it is synthesized. A low-pass filter (so called because it passes low spatial frequencies) ensures that this is so. The low-pass filter is an optical diffusing screen mounted right in front of the sensor. It ensures that every ray of light entering the camera, no matter how sharply focused, will spread across more than one pixel on the sensor.
That sounds even more scandalous - building a filter into the camera to blur the image. But we do get good pictures in spite of the low-pass filter, or even because of it. The blur can be overcome by digital image sharpening. The low-pass filter enables Bayer color synthesis and also reduces moire effects that would otherwise result when you photograph a striped object, such as a distant zebra, and its stripes interact with the pixel grid.
Remember that light spreads and diffuses in film, too. Film is translucent, and light can pass through it sideways as well as straight-on. We rely on diffusion to make bright stars look bigger than faint stars. My experience with DSLRs is that, even with their low-pass filters, they have less of this kind of diffusion than most films do.
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