The camera response curve is a graph showing the relationship between the amount of incoming light received by the camera and the tonal value of that light recorded by the camera. The red line, my own Nikon D800E camera response curve created from fifty exposures then plotted in graph form. The green line represents the linear curve if the camera recorded identical tones as the subject. All cameras have almost identical camera response curves.
Cameras change the tones we see. Graphing the camera response curve allows us to quantify those changes. If the camera did capture the tones we see, then the red line would overlay the green. The ‘changes’ being the difference between the two lines. The angle of the red line indicates the level of contrast. Steeper the angle, higher the contrast, greater the tonal separation. Flatter the curve, lower the contrast, more compressed the tonal separation.
Highlight tones above 50%, A ~ B, gain 45% more contrast giving enhanced tonal separation, great for flat skies. But the shadows are killed. Below 50%, B ~ C, there is a 53% reduction in contrast and tonal separation. The darker the shadow tone, the worse it becomes – 50% ~ 75% shadows lose 26% contrast but from 75% ~ 100% a whopping 79% reduction in contrast and separation! We cannot create rich shadows with these numbers, we must overexpose the shadows.