Level 0

The difference of light and dark is very high.

We can see the dark area and where light source hit clearly

Level 1

Slightly improving light situation in dark areas.

Brighten up the dark area so details in them can be see clearer.

Level 2

Larger dark areas is brightened equally to the general brightness of the image.

The images look balance in term of exposure, not too much bright or dark.

Level 3

Brightening other smaller dark areas as well as object shadow. The light is almost even but we still see the main light source.

Level 4

Even the light as much as we can. There is no difference between light and dark.

This extreme level is not desirable for most styles and types of properties.