Desaturate using main panel saturation sliders or HSL sliders

[Bryan Conner]

When I have a raw image open in ACR 4.6, I can desaturate the image by either using the main panel's saturation slider set to -100 or by setting each color's saturation slider to -100 in the HSL panel.  When I compare the results of these two methods, they are different.  It is a distinct difference both visually and when viewing the ACR histograms as seen below.  I am merely curious, as to why is there such a difference in the contrast between the two methods.  Thanks for your time if you have the answer or an idea.

[walter.sk]

Could it be that the Saturation slider in the Basic tab sets R, G & B values to 0, while the HSL tab sliders when all set to 0 are operating in the LAB space?  Notice also that you get still a different contrast and histogram if you check the Grayscale box on the HSL tab.

[digitaldog]

There’s no Lab going on in this product.

There are differences in the various methods. You can select grayscale in HLS but you lose some tools above (but there are tools below to use). See: http://lightroom-news.com/wp-content/uploads/2009/03/grayscalemixer1.mov

Or you could move all the S in HLS to zero and have Vibrance and Saturation along with Camera Calibration. Lots of possibilities here. All different.

[Bryan Conner]

Thanks Walter and Andrew for your comments. 

Andrew, thanks for the link to Martin's video.  That was a very informative video.  I really enjoy Martin's info, as I do yours, but since I do not use Lightroom, I had never seen it.  I guess I should branch out a bit in my education....LOL.  The different renderings of the image produced when using the main panel saturation slider set to -100 compared to the HSL sliders at -100 in reality are simply options that I can be aware of and select from in order to achieve the look I am seeing in my mind for the particular image.  The artist part of me is screaming "don't wonder why.....just use it!", but the techie nerd/little boy in me still wants to know "why?".