|The "DR Custom grayscale" technique:
Preamble. The range of values I give below work well with images that start as full scale (RGB 3 to 5 for blacks, and 247 to 252 for whites) Colormatch
RGB images. If you are starting with images that are in Adobe RGB, or full out 0 to 255 values you may need to "extrapolate" and experiment.
1. Change from RGB colorspace to LAB color space.
2. Choose the lightness layer (from the Channels palette), from the IMAGE menu choose CALCULATIONS. A dialog box will appear with a bewildering
number of options. There are essentially four main areas, Source 1, Source 2, Blending, and Result:
I'm going to tell you what to change, the default will be for Source 1 and Source 2 to be the name of the current file. Leave these as they are, also leave the "Layer" selection at "Background." Provided that you have chosen the "Lightness" channel, "Lightness" should be the default selection for "Channel".
Change the "Blending" mode to "Multiply"
Set Opacity to a value between 40 and 60 percent. This will affect the shadow areas disproportionally more than the highlight values. Aim for solid blacks in the darkest parts of your image, with the detailed shadow areas just barely visible.
The "Results" area will be set to "New Channel" as default, leave this as is.
Choose OK, and the result of the calculation will appear as "Alpha 1" in the Channels Palette.
3. Choose the Lightness channel a second time from the Channels Palette.
Choose Calculations from the Image Menu a second time.
We will repeat the operation outlined above EXCEPT with a change in the "Opacity" field (in the blending area) to 5 percent. The result will be a slighly denser lightness channel, but will appear washed out.
Click OK, and the result of the calculation will appear as "Alpha 2" in the Channels Palette.
4. Leave Alpha 2 selected in Channels Palette.
Choose Calculations from the Image Menu a third time.
This time change the Source 1 "channel" to "Alpha 1" and change the Blending mode to "Hardlight"
Set Opacity to between 40 and 50 percent. We are looking for an effect similar to a grade 3 paper as in days of old.
Click OK, and the result of the calculation will appear as "Alpha 3" in the Channels Palette.
For some this may be a result they wish to keep. I usually find it necessary to "temper" the result a bit, so I add one more step.
5. Leave Alpha 3 selected in the Channels Palette.
Choose Calculations from the Image Menu a fourth time.
Change Source 1 to Alpha 2 (the weak washed out image)
Change Blending mode to "Normal"
Set Opacity to "taste" (typically between 40 and 60 percent.
If you are happy with the result, click OK, and the result will appear as "Alpha 4" in the Channel Pallette.
Sometimes you may be getting too dark of a shadow area. If this is the case try reversing the sources, by setting Source 1 to "Alpha 3" and Source 2 to "Alpha2" and play with the opacity setting.
When you are satisfied, convert the result to Grayscale (discard color), and continue on from there.
If you feel that the blue sky should be a little deeper, or what was an orange colored object is too close in value to a light blue object, then try converting to RGB (the "Alpha 4" channel will remain selected).
Inspect the R, G, and B channels. If you are wanting to make the sky darker, you'll find that the R (red) channel will have a much darker result. Lets assume that's what we want for this example.
If so, set Source 1 to the "R"(red) channel
The "Normal" blending mode set at 50 percent opacity may look OK, but you may want to experiment and see what happens with it set to "Darken" or "Overlay" as well.
Sometimes you may wish to lighten a particular color, and that's possible as well. Usually the "lighten" or "screen" mode will be needed in these cases.
Set the Source 1 to the color you wish to lighten and set the blending mode to "lighten" or "screen" and adjust to taste.
As above, when you are satisfied, convert the result to Grayscale (discard color) and save in your choice of file format.
In some very drastic situations (usually when handling imaging chores for a client that supplies their own scans or prints) I've had to resort to doing a separate calculation for one color or tonal value. I then duplicate the image, convert the best channels to grayscale, and then drag the "correcting" layer on top of the other (holding the shift key down to align). I apply a "layer mask" typically the "Hide All" variety, and "paint on the mask with a white brush to change the tonal value of that specific area. It's easier than burning and dodging, as the results are completely reversable, even after saving the image (provided you saved it as a PSD file with alpha channels intact.