Today I have try to calibrate my old Imac with Dispcal. Below the results: the average and maximum error at the end are worse than at beginning. Why?
Possibly because each round uses twice the number of test points of the previous round, and so it is looking for values it hasn't found before (calibrating parts of the neutral curve it hasn't looked at before). Display drift during calibration can also have this effect. [Attempting to get perfect neutrals near black is basically impossible in many situations because the control is limited (typically 8 bit precision) and the display and instrument have poor repeatability near black. This is compensated by the fact that it's so dark that it's hard to see any hue. ] Graeme Gill.