Consider the following photograph, taken on a drizzly day in Norway with a cloudy sky, and the mountains somewhat obscured by mist and clouds.

Now let’s look at the intensity image (the colour image has been converted to 8-bit monochrome):

If we look at a region near the top of the mountain, and extract a circular region, there are three distinct regions along a line. To the human eye, these appear as quite uniform regions, which transition along a crisp border. In the profile of a line through these regions though, there are two “cliffs” (Aand B) that marks the shift from one region to the next. Human eyes don’t perceive these “cliffs”.

The Mach bands is an illusion that suggests edges in an image where in fact the intensity is changing in a smooth manner.

The downside to Mach bands is that they are an artificial phenomena produced by the human visual system. As such, it might actually interfere with visual inspection to determine the sharpness contained in an image.