Grayscale/Brightness Resolution vs Contrast
Gray-scale resolution and image contrast are different.

Gray-scale resolution looks at how many gray scales are being used to present the image.

Image contrast, on the other hand, looks at the difference between the maximum and minimum values of the gray scales used to present an image.

For example, a low gray-scale resolution, low contrast image might only use #000000 and #010101 (#RRGGBB where RR = red value, GG = green value, BB = blue value).

A low gray-scale resolution, high contrast image might only use #000000 and #FFFFFF (and only those two gray-scale values).

A high gray-scale resolution, low contrast image might have a maximum gray scale value of #1F1F1F (5 bits) and a minimum of #000000, but use all gray scales in between.

A high gray-scale resolution, high contrast image might have a maximum gray scale value of #FFFFFF (8 bits) and a minimum of #000000 (and use all gray scales in between).