This was an interesting exercise that I have not done before. I had read that in automatic modes modern DSLRs find it hard to differentiate different shades so that even black and white can come out looking the same, and that is exactly what has happened:
These were shots of, respectively, black, grey, and white cards. Although there are some slight differences the histograms are very similar. All were shot from the same position, the camera mounted on a tripod, focused manually by reference to the distance scale on the lens (50mm 1/1.4) – not surprisingly the autofocus would not work. I know for the purpose of this exercise it is not necessary to focus accurately but as I was hoping that the second set, shot manually, would bring out more of the qualities of the ‘neutral’ grey card I decided it would be useful to focus as accurately as I could.
The camera settings chosen by the camera were unsurprisingly different – black, f/2.8, 1/60s, ISO 320; grey, f/3.5, 1/125s, ISO 100; white, f/4.5, 1/160s, ISO 100 – nevertheless the end results are barely distinguishable (but for a blemish on the grey card!).
Switching to Manual I set the camera to f/22 for all the shots, otherwise exactly the same procedure as above. This time the images are distinctly different and the different shades are clearly apparent.
To get the right histograms all I had to do was adjust the shutter speed for the white card to 1.30s. The other two were both set at 0.50s. ISO for all three was 100.
All the shots were taken in natural light, somewhat overcast, so perhaps it is not great surprise that the second set have much slower shutter speeds than the first. I did notice while experimenting in camera before taking the shots that setting the shutter speed for the white card to 0.50s as well produced a histogram with he peak shifted significantly back towards the centre, about one third of the way in from the right.
The other thing I find particularly striking is that the grey card shows its colours and texture much more clearly; it is not a monotone, nor is it smooth. The black and white cards are, in contrast, monotones and the surfaces smooth.