Let's start with a scene featuring an abrupt change in brightness, such as the one below, showing a coffee cup in full sunlight and its shadow. In practice, we take a digitized JPEG photograph of the scene at 300 dpi (that is, three hundred pixels per inch) and convert it into a much "lighter" 36 dpi file.

The coffee cup at 300 dpi.

The coffee cup at 36 dpi.