Digital BW, The Print

I think you are right.  With QTR Gray Lab there is no luminance shift.
Seems odd though.  I would expect the single channel to still be displayed
in the document's (colour) space or at the very least converted to the grey
workspace rather than assigned this different space.  If I drag a layer from
one document to the another in a different colour space I believe the layer
is converted to the new space.  At any rate I found the gamma change from
the assignment of Gray Gamma 2.2 rather than Lab to be quite beneficial -
much more "pop".


> From: John Moody <moodymz3@...>
> Reply-To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Date: Thu, 10 Nov 2005 08:53:45 -0500
> To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Subject: RE: [Digital BW] Channel Mixer and Lab L Channel
> 
> Steve,
> I believe when you turn off the a and b channel, PS displays the image based
> on your working gray space.  The same thing probably happened when you
> brought the L channel over, it got converted to your working gray.
> Try setting your working gray to Lab grayscale and see if the results are
> closer to what you were expecting.
> 
> Best regards,
> John Moody
> 
> -----Original Message-----
> From: DigitalBlackandWhiteThePrint@yahoogroups.com
> [mailto:DigitalBlackandWhiteThePrint@yahoogroups.com]On Behalf Of Steve Kale
> Sent: Thursday, November 10, 2005 6:00 AM
> To: DigitalBlackandWhiteThePrint@yahoogroups.com
> Subject: [Digital BW] Channel Mixer and Lab L Channel
> 
> 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.)