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Tone Management System - Lab

 

Introduction

The methods described here retain tonality as defined by the Lightness channel of the Lab color space.  An image can be quickly converted to Lab in Adobe Photoshop by clicking Image > Mode > Lab Color.  We can then go to the Layers panel and turn off channels a and b.  The result is a black and white image.  However, we do lose the ability to adjust tone by color.  The methods I devised and discuss here overcome this disadvantage while maintaining Lab as the desired tonality.

 

Tonal Adjustments By Color

The three basic Photoshop controls for tonal adjustments are Hue/Saturation, Levels and Curves.  Generally, it is best to use Levels or Curves for tonal adjustments.

We can use all three Photoshop controls to adjust tone by color ranges.  We can use Levels and Curves to adjust tone by color channel.

This section of the web site does not go into the details of how to use Hue/Saturation, Levels or Curves to make tonal adjustments.  That information is covered on the Hue/Saturation page, the Levels page and the Curves page.  For in-depth information on how to use Levels or Curves with color ranges, read the Using Levels and Curves with Color Ranges page.


The first example uses Curves for tonal adjustments by color.  The second example uses Levels.

 

LCC Method

The LCC method preserves Lab luminance, uses a Channel Mixer adjustment layer for monochrome conversion, and a Curves adjustment layer for tone management by color.  The step-by-step instructions to use this method are as follows.
 

Copy Original Image

After all global and local changes have been made …

  1. Click Image > Duplicate.  Type the name Lab RGB and click OK.  If desired, another name can be used.
  2. Make sure Lab RGB is the active document and click Layer > Flatten Image.  If asked to discard hidden layers, then there are layers in the Lab RGB document that are not being used.  These layers will be in the Layers panel and they will not have an eye icon Eye icon.  If these layers are not going to be used, click OK.  Otherwise, click cancel and activate the layer in both the Lab RGB and original documents and repeat step 2.
  3. Save and close the original document.

Channels panel
Figure 1.  Lightness Channel of a Lab Image

 

Luminosity blending mode
Figure 2.  Using The Lab Lightness Channel as a Luminosity Filter

 

Create Lab Filter

At this point, we now have a copy of the original image with all global and local changes applied.  This new document will become the final black and white RGB image.  The reason it has to be copied is to avoid double applying the global and local changes when we apply the Lab 'filter'.

  1. Click Image > Duplicate. Type the name Lab and click OK. Another copy will be created. This second copy is temporary and will be discarded shortly.
  2. Make sure the Lab document is the active document.
  3. Click Image > Mode > Lab Color.
  4. Rearrange the Lab RGB and the Lab documents on the monitor so that both can be seen.
  5. Make sure the Lab document is the active document.
  6. Click the Channels panel.  We should see four channels: Lab, Lightness, a and b.
  7. Make the Lightness channel the active channel and turn off the other three channels by turning off their eye icon Eye icon.  See Figure 1.
  8. Click Select > All.
  9. Click Edit > Copy.
  10. Make Lab RGB the active document.
  11. Make sure the Layers panel is the active panel.
  12. Click Layer > New > Layer.  Name the layer Lab and click OK.
  13. Click Edit > Paste.
  14. Looking at the Layers panel, if the Lab layer is not the topmost layer, drag it to the top.
  15. In the Lab RGB document, change the blending mode of the Lab layer to Luminosity.  Do not be concerned when the colors reappear.  We are using the Lab layer to control luminance, not color.  We will remove the color later.  The Layers panel for the Lab RGB document should look like Figure 2.  Note the blending mode of the Lab layer.
  16. Close, without saving, the Lab document.

 

At this point, the only open document should be Lab RGB and its tonality matches that of a true Lab image.


 

Convert to Black and White

Channel mixer
Figure 3.  Channel Mixer Settings

We will now create the necessary adjustment layer to convert the image to monochrome and allow us to control tone by color.
 

Above the image and above the Lab layer...

  1. Create a Channel Mixer adjustment layer by clicking Layer > New Adjustment Layer > Channel Mixer.  Name the layer as desired.
  2. Set the Red channel to 30%.
  3. Set the Green channel to 59%.
  4. Set the Blue channel to 11%.
  5. Leave Constant 0%.
  6. Check the Monochrome check box.  See Figure 3 for the proper Channel Mixer settings.
  7. Click OK.
  8. Leave blending mode Normal.  For now, turn off the Channel Mixer's visibility by turning off its eye icon Eye icon in the Layers panel.

 

Tip

On the Downloads page is a Photoshop Action that will perform steps 1 through 27 (except step 3) for the photographer.

 

adjust tone by color range

  1. Click Select > Color Range and use the eyedropper tools to select the desired colors. Click OK to create the selection.
  2. Below the Channel Mixer adjustment layer and above the Lab layer, create a Curves adjustment layer by clicking Layer > New Adjustment Layer > Curves.
  3. Click OK without making any changes.  The selection created in step 28 is now a layer mask.
  4. Change the blending mode of the Curves adjustment layer to Luminosity.
  5. Reopen the Curves adjustment layer.
  6. Make the necessary tonal adjustments.  Check the results by turning the Channel Mixer's visibility on and off.  For in-depth information on how to use Curves to make tonal adjustments, read the Curves Adjustments page.  For additional information on using color ranges with Curves, read the Using Levels and Curves with Color Ranges page.

 

Or, adjust tone by color channel

Curves dialog box
Figure 4.  Curves Dialog Box

  1. Below the Channel Mixer adjustment layer and above the Lab layer, create a Curves adjustment layer by clicking Layer > New Adjustment Layer > Curves.
  2. Click OK without making any changes.
  3. Change the blending mode of the Curves adjustment layer to Luminosity.
  4. Reopen the Curves adjustment layer.
  5. Using the Channel drop down box in the Curves dialog box (see Figure 4), select the desired channel.
  6. Make the necessary tonal adjustments.  Check the results by turning the Channel Mixer's visibility on and off.  For in-depth information on how to use Curves to make tonal adjustment, read the Curves Adjustments page.

 

Steps 20 through 27 convert the color RGB image to a black and white RGB image.  The percentages in steps 21 through 23 are critical for preserving the luminosity from the Lab layer.  Do not change these percentages.  Instead, to change an image's tone, use the Levels or Curves adjustments.  Steps 31 and 36 are critical to ensure the changes we make only affect tone, not color.  Steps 28 through 33, or steps 34 through 39, are where we manage an image's tone by color.
 

Make non-color based tonal adjustments

  1. Above the Channel Mixer adjustment layer, create a Levels or Curves adjustment layer by clicking Layer > New Adjustment Layer > Levels/Curves.
  2. Make the desired adjustments and click OK.
  3. Leave blending mode Normal.

 

Summary

Layers panel
Figure 5.  LCC Layers

Figure 5 is an example of what the Layers panel for the LCC method should look like if done properly.  Note the blending mode of the Curves adjustment layer.

 

Like the method that retains grayscale tones, this method appears to be rather involved.  Especially with creating two copies of our image.  However, if the Lab layer were created in the original image, then any global and local changes will be applied twice.  Once as part of the original image and again when the image was flattened and then copied back to the original.

By changing the blending mode of the Lab layer, we turned it into a luminosity filter.  Basically, it allows the color to pass through but replaces the luminance in the underlying image with Lab luminance.  Since color still passes through, we can manage tone by color while retaining Lab luminance.  The Channel Mixer layer does not replace the Lab luminance with its own.  Rather it preserves the Lab luminance.

The advantages of the LCC method are many.  For those who prefer Lab tone, we preserve Lab tonality while maintaining the ability to adjust tone by color.  The method is non-destructive and since we are using Curves and/or Levels for tonal adjustments, it is predictable.

There are three main disadvantages.  First, we end up with two images.  Our master image and a black and white RGB version of it.  Second, if we decide to go back and change any of the global and/or local adjustments on the master image, we will have to recreate the black and white RGB image from the beginning.  So it is best to be sure we are done with the color version of the image before working on the black and white version.  Third, because of the number of steps involved, it is not easily intuitive.


LHL Method

The second method to be discussed is the LHL method.  The LHL method preserves Lab luminance, uses a Hue/Saturation adjustment layer for monochrome conversion, and a  Levels adjustment layer for tone management by color.  The step-by-step instructions to use this method are as follows.

 

Create Lab layer

Hue/Saturation
Figure 6.  Hue/Saturation to Remove Color

  1. Follow steps 1 through 19 above to create a copy of the image with a Lab layer with a blending mode of luminosity.

 

Create monochrome conversion layer

Above both the image and Lab layers...

  1. Create a Hue/Saturation adjustment layer by clicking Layer > New Adjustment Layer > Hue/Saturation.
  2. Make sure the Edit drop down box is Master and set Saturation to -100.  See Figure 6.
  3. Click OK.
  4. Change the blending mode to Color.  For now, turn off the Hue/Saturation's visibility by turning off its eye icon Eye icon in the Layers panel.

 

Tip

On the Downloads page is a Photoshop Action that will perform this conversion for the photographer.

 

adjust tone by color range

  1. Click Select > Color Range and use the eyedropper tools to select the desired colors. Click OK to create the selection.
  2. Below the Hue/Saturation adjustment layer and above the Lab layer, create a Levels adjustment layer by clicking Layer > New Adjustment Layer > Levels.
  3. Click OK without making any changes.  The selection created in step 5 is now a layer mask.
  4. Change the blending mode of the Levels adjustment layer to Luminosity.
  5. Reopen the Levels adjustment layer.
  6. Make the necessary tonal adjustments.  Check the results by turning the Hue/Saturation's visibility on and off.  For in-depth information on how to use Levels to make tonal adjustments, read the Levels page.  For additional information on using color ranges with Levels, read the Using Levels and Curves with Color Ranges page.

 

Or, adjust tone by color channel

Levels dialog box
Figure 7.  Levels Dialog Box

  1. Below the Hue/Saturation adjustment layer and above the Lab layer, create a Levels adjustment layer by clicking Layer > New Adjustment Layer > Levels.
  2. Click OK without making any changes.
  3. Change the blending mode of the Levels adjustment layer to Luminosity.
  4. Reopen the Levels adjustment layer.
  5. Using the Channel drop down box in the Levels dialog box (see Figure 7), select the desired channel.
  6. Make the necessary tonal adjustments.  Check the results by turning the Hue/Saturation's visibility on and off.  For in-depth information on how to use Levels to make tonal adjustment, read the Levels page.

 

Steps 2 through 5 convert the color RGB image to a black and white RGB image.  Step 5 is critical to limit the desaturation adjustment to color, not tone.  If we do not change the blending mode to Color, Photoshop will calculate the resulting tone using a different algorithm.  (More information about this can be found on the Customary Method - Desaturation page.)  By changing the blending mode to Color, we drastically change the way the Hue/Saturation adjustment layer desaturates an image.  The Color blending mode causes Photoshop to remove the color with affecting luminance.  Steps 5 through 10, or steps 11 through 16, are where we manage an image's tone by color.

 

Make non-color based tonal adjustments

Layers panel
Figure 8.  LHL Layers

  1. Above the Hue/Saturation adjustment layer, create a Levels or Curves adjustment layer by clicking Layer > New Adjustment Layer > Levels/Curves.
  2. Make the desired adjustments and click OK.
  3. Leave blending mode Normal.

 

Summary

The adjustment layers below the Hue/Saturation adjustment layer allow us to adjust tone by color range or color channel.  Since the image is black and white 'above' the Hue/Saturation adjustment layer, we cannot use color to target our tonal adjustments above this layer.  Therefore, any tonal adjustments we need to make that are not based on color can be made above the Hue/Saturation adjustment layer.

Figure 8 is an example of what the Layers panel for the LHL method should look like if done properly.  Note the blending mode for the Hue/Saturation adjustment layer.

The advantages of the LHL method are many.  Lab luminance is accurately retained.  We still have control over tonality by color.  The method is non-destructive and predictable.


Figures 9A and 9B show before and after images using either the LCx or LHx methods.

Color image Figure 9A.  The Color RGB Image

Black and White Figure 9B.  The black and white version using either the LCx or the LHx methods

Tip

Once an image has been converted using LCx or LHx, we can go to the Photoshop Layers panel and click the eye icon Eye icon on and off on the Lab layer.  When the eye icon is on, the base luminance is Lab.  Off, it is measured luminance.

 


Which Adjustment Layer for Monochrome Conversion?

The LCx and the LHx methods give almost identical results.  In my tests, converting to monochrome using the Hue/Saturation adjustment layer (LHx), when used with the correct blending mode of Color, gives slightly more accurate results than Channel Mixer (LCx).  However, the difference is imperceptible (around 1/25 of a stop).  Therefore, we can use either one.  The critical steps are getting the adjustment layers in the proper order and getting the blending modes correct.

 

Which Adjustment Layer for Tone Control by Color?

The first method above uses a Curves adjustment layer to manage tones by color.  The second method uses a Levels adjustment layer.  Which is better?  The one you are more comfortable with.  Levels has the advantage of giving us a histogram so we can readily compare the image's tonal range to the digital tonal range.  Curves has the advantage of giving us more control along the entire tonal range.  Levels and Curves can easily be used to make adjustments by color channel.  With a little bit of additional work, they can also be used with color ranges.  Hue/Saturation gives us the least amount of control over the tonal range, but it is the easiest control to use when working with color ranges.  Hue/Saturation cannot be used with color channels.