Explanation of black and white points

[Bill WEber]

Could someone please tell me in the most technical terms possible, what the effect is of setting the black and white points? Why does this act "expand the tonal range" of the photo? What does it do for the histogram? What does it do for the possible color gamut in the photo? Thank you.

[Manfred M]

Welcome to CiC Bill. I'm not sure what you mean by the "most technical terms possible". I think the best way to demonstrate / explain is to do this with an example and some discussions around the example.

I've taken an image that has a tonal range that has a luminosity histogram that does not show values that go from 0 (black) to 255 (white) even though the scene has areas (shadows) that are black and areas that are white (snow on the mountain). The following image shows the histogram that Photoshop CC 2017 shows for this image.






The second image shows the black point and white points set in the way that is usually recommended, i.e. the place where the values of the histogram start ramping up. Here I have moved the black to a value of 27 and the white point to 219. The tonal range and contrast of the image have increased





I've shown what the luminosity histogram looks like for the modified image. By doing so, all the values in the image that have a value of 0 - 27 have been set to black and all the values from 219 - 255 have been set to white. The values in between these two points are remapped to new values. If I were to look at the histograms of the individual colour channels, I would see changes there that correspond to the new values. I believe the mapping of the individual colour channels to the luminosity value is (Red x 0.30) + (Green x 0.59) + (Blue x 0.11).




I'm not quite sure what your question about the colour gamut is coming from. Gamut is a measurement of how well a specific device maps to a particular colour space, so I think the question does not make any sense to me.