Optimal image histogram for Human eye
Human eye perception is a complicated thing, the human perception tend to fail in different situations even in large changes of gray tones, also is capable of note minimal changes in the gray levels but all depends on the shape of the figure rather than its histogram.

In some cases you can enhance an image by applying histogram equalization techniques but this is only to emphasize some gray tones in order to observe more details in an image. This is related to gray images.