Lab Mode Values incorrect

[jdf-i]

Hello,

How do I correct my Lab Mode color Values?

I am switching to a new laptop and my Lab Mode color values are incorrect. For instance Gray (rgb 128) is reading as L=61 not the L=54 that has been the default conversion. The a and b values are also off.

My old laptop running Photoshop Version 21.2.4 is still displaying the correct values.

Thanks in advance for any help including suggestions as to where to list this post.

[Francesco Della Peruta]

Hi, I think that the conversion RGB -> lab depens on the active color profile you are using.

Please double check Color managment settings between the Macs.

[Norman Sanders]

In the first edition of Dan Margulis' Photoshop Lab Color, he touches on this subject on page 99 with this comment related to the midpoint Lab L value and RGB(128) by noting that "...the higher the gamma, the darker the midpoint. (My emphasis) Plus, we need to know that almost everyone uses one of two gamma settings: 1.8, which is the tradaditional Macintosh setting, or 2.2, the traditional PC setting. Both sRGB and Adobe RGB both employ 2.2...."

There's more, where the L value is set from 54 to 61 (the variable is never at 50). Dan delves deeper into the subject in Chapter 13, as well. 

So, for starters, I would strongly recommend that first edition. Its ISBN No. is 0-321-35678-0

BTW: If you are interested in going deep into Lab Color, his later edition is just as valuable.

[jdf-i]

Thanks all!

It is the Gamma.

And thanks for pointing me to Dan's book. I pulled out my dog-eared copy and and went right to page 99!

Again thank you very much for solving my problem so quickly.

[D Fosse · Accepted solution]

In short, it depends on the source RGB color space.

I'm not at my workstation now to test, but it sounds like the difference between sRGB/Adobe RGB and ProPhoto.

Adobe RGB has a regular 2.2 gamma, ProPhoto has 1.8.

sRGB has a more complex and irregular tone curve that can't be described as a clean gamma function, but it averages out somewhere around 2.2.