H Histogram 139 A histogram is a bar chart that displays the numbers of pixels in each colour intensity level. The simple histogram usually shows 256 levels of brightness from 0 to 255 where black is 0, mid-grey is 128 and white is level 255. White is usually to the right of the graph with all the light tones to the right and above mid-grey and all the dark tones falling to the left. Vertical height is unimportant as the category with the biggest pixel count is made to fit the maximum vertical height on the display. Some camera histograms are divided up to show a `safe' five-stop range. There is no such thing as a perfect histogram ­ the shape of the chart depends entirely on what is in front of the camera. Dark subjects will show a bias to the left, light subjects a bias to the right, high-contrast subjects a classic U-shaped chart and average subjects a central hump shape. Some cameras show red, green and blue channels independently.