Mid grey - what value ought a mid grey tone have?
The bitter truth is that is it depends on the colour space you are using.
Lab = 50 0 0
ProPhoto = 100
Adobe = 118
sRBG = 119
Linear = 128
For all except Lab, R = G = B, i.e. in sRGB, mid grey is 119, 119, 119.
Have fun!
Claes in Lund, Sweden
Morning, James!
Yes, that is an interesting link.
However, my present work/idea/play deals with using a grey card not as a tool for proper exposure, but as something to make postprocessing a trifle closer to reality 
Have fun!
Claes in Lund, Sweden
I’m not sure what you mean by “Linear”. Usually “linear” refers to an RGB working color space with a linear gamma TRC, whether the color space by its specs has a linear gamma TRC (eg ACES), or perhaps is a linear gamma variant of a color space that by its specs has a nonlinear TRC (eg linear gamma versions of sRGB or ProPhotoRGB).
For RGB working color spaces with a linear gamma TRC, middle gray is R=G=B=0.18 floating point, or on a scale from 0 to 255, R=G=B=47.
For RGB working color spaces that have the LAB L companding curve, middle gray is R=G=B=0.5 floating point, or on a scale from 0 to 255, middle gray is 128 (rounded from 127.5).
Evening, ma’am,
Theoretically, you are much more proficient than I in these matters. I am just an experimenting techie.
I first wrote about how to use a SpyderCube in post processing in 2013, and I faintly recall that Bruce Lindbloom’s work influenced me at the time and that I got the term Linear from him.
Have fun!
Claes in Lund, Sweden
Hmm, I’m not sure what you mean by theoretically. Everything I think I know about color management and color mixing is based on experimenting and testing - lots of experimenting, backed up by reading and more experimenting. Almost all such experimenting and testing is in the context of using color management and color mixing in the digital darkroom. Of course I still make mistakes! That’s how we learn!
That was my question. I was wondering how you connected “Linear” with a value of 128 for middle gray. Below is an image that shows the practical difference between the various TRCs, by comparing gradients drawn in various RGB working spaces:
Middle gray at 128 is what the middle patch in the second row in the image above shows. The image demonstrates everything you noted about middle gray in the various RGB working spaces, except for the “linear” bit, which it’s not clear exactly what you mean. If you mean “linear gamma RGB color spaces have middle gray at 128”, then your value for middle gray is wrong. But if you are using “linear” to mean “corresponding linearly to how we perceive changes in relative luminance”, then your terminology is perhaps misleading, but your value for middle gray is correct.
My apologies for being overly technical 
.
Evening, Claes,
Sorry, I did not mean to imply anything about your use of grey cards. Reading the 18% figure in Morgan’s post reminded me about the 18% myth, which I thought was interesting. Perhaps it would have been more relevant on your grey card thread.
And I was lazy, posting the link without further words 