GRADIENT MAP ADJUSTMENT LAYERS

My favorite use of a Gradient Map adjustment layer is in making "luminosity" selections. With a luminosity selection, you can apply adjustments to your image selectively, and learning how to use a Gradient Map adjustment layer can make luminosity selections even more powerful.  Even if you don't care about luminosity selections, this tutorial will teach you the basics of how to use a gradient map adjustment layer, which you can then apply in other ways.

So, here’s how to use a Gradient Map adjustment layer.

1) Open up an image and duplicate the background layer:

2) Set the foreground and background color swatches on the floating tool bar to the defaults by hitting 'd' on your keyboard.

3)Click on Layer>New Adjustment Layer>Gradient Map, check "Group With Previous Layer", and click OK. The Gradient Map dialog box will appear. Click in the middle of the gradient bar displayed under the heading "Gradient used for Greyscale Mapping".

That will open up the Gradient Editor:

The default is a foreground to background gradient which in this case is a black to white gradient. The chosen gradient is displayed in the gradient bar:

The little squares along the bottom of the gradient bar are called "color stops".  The left color stop corresponds to 0% brightness in your image, and the right color stop corresponds to 100% brightness in your image. That never changes, and it's your point of reference when using gradients on your image: the shadows in your image are changed with the left side of the gradient bar and the highlights are changed with the right side of the gradient bar. 

Click on the left color stop to see the information for that stop:

Note: the active color stop is the one with the filled in black triangle touching it's top edge. The information for the active color stop shows its color and location in terms of % brightness. The color stops are used to change the colors in your image, and the brightness values associated with each stop determine which areas in your image will get converted to the color indicated by the color stop.

For this tutorial, I'm going to isolate the midtones of an image by turning them white and by turning the highlights and shadows black. The shadows encompass the range 0-32% brightness, the midtones 33-66% brightness, and the highlights 67-100% brightness. I'm going to set additional color stops at 33% and 66% brightness, which are the dividing points for each range.

To set a color stop all you have to do is click below the gradient bar. When your cursor changes to a hand icon, you can set a color stop by left clicking your mouse. To move the color stop, drag it to the left or right, or enter a value next to "Location:". To delete a color stop, drag it downwards off the gradient bar. To change the color of a color stop click the right arrow next to the color swatch and select either foreground or background color(easiest), or left click on the color swatch next to "Color:", which will launch the Color Picker dialog box.

4) Add a color stop at location 33% and 66% brightness and set both of their colors to white. 

5) Set the color of the right color stop(location 100%) to black. 

Here is what the gradient bar should look like:

If you look at the effect on your image, you can see how the image was recolored according to the location and color of the color stops:

The midtones in the image corresponding to the brightness range 33-66% were turned white, and the shadows and the highlights were turned black.  In addition, the transitions between the midtones and the shadows and highlights were faded into each other.  The way the ranges are made to fade into each other is shown graphically on the gradient bar.

When you add a color stop, you may have noticed the small diamonds that appear between color stops. They control the fade between the colors of your gradient, and you can slide them to the right or left to adjust the fade.  If you don't want the colors to fade into each other and you would rather have sharp transitions, add additional color stops right next to the existing ones:

Finally, the squares on top of the gradient bar are called "opacity stops". They allow you to adjust the opacity of the gradient colors that are applied to the different parts of your image. If you think of a gradient map adjustment layer as painting the colors indicated by the color stops on top of your image, then the opacity stops allow you to apply more or less of the color to the image.  They will NOT make any part of your image transparent--the extent of what you can do with an opacity stop is apply 0% of the gradient color, which leaves the underlying image unchanged.  If you add an opacity stop and change it's opacity to 0% and slide it around, you can readily see how it affects the gradient bar.* 

5)Click OK, OK to close the two dialog boxes.  Then, merge the Gradient Map adjustment layer into the Background copy layer(ctrl+E).

Now you are ready to use the black and white representation of your image that you created to make a luminosity selection--that will concentrate the effect of any adjustment you make on the midtones.

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*Unfortunately, if you look at your image while sliding the opacity stop around, you should notice that it doesn't do anything to the image.  That appears to be a bug(at least in PSE2).  You can also verify that your image isn't affected by the opacity stops by dragging the Gradient Editor dialog box to the side to reveal the Gradient Map dialog box.  If you set an opacity stop with 0% opacity in the Gradient Editor, and you slide it back and forth while looking at the Gradient Map dialog box, you can see that the gradient displayed under the heading "Gradient used for Grayscale Mapping" doesn't change.  However, knowing how opacity stops work is still useful if you ever plan on using the Gradient tool.  If you click on the Gradient tool and then click on the gradient in the options bar, you will see that adding opacity stops does affect the gradient displayed in the options bar.  The gradient displayed in the options bar is then applied when you use the Gradient tool.