Digital BW, The Print

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, "Steve Kale"
<stevekale@b...> wrote:
>
> I am puzzled by something I stumbled across yesterday.  I was
working up an image 
> converted to B&W with Channel Mixer (60% Green, 40% Blue).  I wanted
to sharpen the 
> image and tried a variation of the Smart Sharpen/Reduce Noise
Sharpen action I use - 
> specifically I copied the image, converted it to Lab Colour Mode
(flattening in the process) 
> and only sharpened the L channel.  I made a mistake while doing this
and brought just the 
> L channel of the copy over to the original as a layer.  This
revealed something I found 
> surprising:  the Lab Colour copy ( a flattened greyscale image) had
luminance information 
> in the a and b channels.  That is the luminance of the L channel
alone was different from 
> the luminance when all three (L, a and b) are displayed.  This would
seem to fly in the face 
> of what I thought the a and b channels represented.  
> 
> I did another quick test.  I took the RGB original with the Channel
Mixer layer and flattened 
> it, leaving a greyscale image in RGB.  Converted it to Lab.  If I
uncheck either one of the a 
> and b channels the image stays looking the same.  But if I uncheck
both the a and b 
> channels (leaving just the Lightness channel displayed) the
luminance of the image 
> changes significantly.
> 
> I must be missing something basic but I don't quite get it.... Ideas?
> 
> Steve
> 
> PS:  I am using PS CS2.  (My RGB space is ProPhoto RGB.)
>

L will always be lighter than grayscale. LUTs are set up this way. You
can check with a color picker. In a space with gamma 2.2 midtone
128,128,128 translates into L 54 while in gamma 1.8 into L 61 if i
remember correctly. So using L for masks or sharpening doesn't give
exactly the same results as using grayscale or reverting sharpening to
luminosity in RGB. Regards,

Pawel