Can someone explain to me, why the exposure module and color picker in darktable is doing the calculation **not** linear and therefor **not** correct? Please correct me, if I am wrong or do miss something basic with my following interpretation.

In the next graphic you see a 50 % checkerboard in the background. In the foreground are two homogenously filled areas. With zoom 100 % the background should have the average intensity of 50 % and therefor roughly sRGB=186 (right side area) and **not** the left side value of sRGB=128.

The color picker from top to bottom does give the results:

The 2nd value of the checkerboard is the arithmetic mean, but **not** the perceived brightness of 186. If you zoom such image in a browser or irfanview or most other tools (inclusive darktable), the brightness of the checkerboard will change and deviate strongly from the correct average of 186. This effect is well known. Look for the good and famous article of www.ericbrasseur.org/gamma.html about gamma error in picture scaling.

But it turns even worse!

If you decrease with the exposure module in darktable with â1 EV, the result is as follows:

The color picker does show (again left to right = top to down):

Again, the mean calculation of the second checkerboard value is the âmeanâ average. That is acceptable as *arithmetic* value, but: The exposure module does the calculation **not** with linear light. The modification of â1 EV corresponds to 50 % intensity. And 50 % (â1 EV) intensity decrease of sRGB=186 (50 % intensity) is not sRGB=93, but it is sRGB=137 (25 % intensity). The value of 93 is too dark, because sRGB=93 is only about 11 % intensity.

You can try this also with any 255 (100 % intensity) white filled area. Decreasing the exposure by â1 EV should give 186 (50 % intensity) and not 127 (21 %). BTW, depending on calculation and rounding with exponent 2.2 (approximation) or 2.4 (accurate) according to sRGB standard the 50 % gray is 196,187 or 188 and 21 % can be 127 or 128. I usually calculate with correct values 188 and 127, but my above figures are not yet updated with this.

Wrong gamma calculations generally give too dark results. Especially with fine bright details this is visible. This because one white pixel line and one black pixel line do NOT give sRGB=127 as perceived average, but sRGB=188, which is much brighter (50 % instead of 21 %).

Looking forward to your enlightening comments.

Thank you!