Response to Zone system and photoshop density readings

Each make and model of scanner will be different, and they'll probably vary from sample to sample as well. Most scanner software applies a gamma correction which will make the density scale non-linear. In addition, if you're very unlucky, the software will also apply an automatic correction for the brightness range of the scan, rendering the results useless.
If you calibrate your individual scanner carefully to a known density wedge, and ensure that the software doesn't make adjustments that you can't control, then it's perfectly possible to get approximate reflective density readings, but there won't be much resolution in the shadows areas. Typically, a 0.3 density change in the dark region only causes a 3 to 6 level change in pixel value.

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