An adjustment layer is a flexible way to add an adjustment to an image without directly changing the image itself. Every adjustment layer comes with a built-in layer mask. You can add black, white, or gray to an adjustment layer mask to target the adjustment to a particular part of a photo.

Add an adjustment layer

1. Select an image layer in the Layers panel.
2. Click the Create new fill or adjustment layer button at the bottom of the Layers panel. Choose Hue/Saturation in the pop-up menu. A Hue/Saturation adjustment layer, with a white layer mask thumbnail, appears in the Layers panel above the layer you selected. The Properties panel displays Hue/Saturation controls for this adjustment layer.
3. In the Properties panel, drag the Saturation slider to the right to increase color saturation. This adjustment now affects the entire photo on the layer below.

Add a black to white gradient to the adjustment layer mask

Black on an adjustment layer mask hides the adjustment; gray partially shows the adjustment; and white shows the adjustment. One way to add black, gray, and white to any layer mask, including an adjustment layer mask, is with a black to white gradient.

1. In the Layers panel, make sure there is a white border around the layer mask thumbnail. If there is not a white border, click the layer mask thumbnail.
2. Press D on the keyboard to set the default colors of white and black in the Toolbar. Press X to switch the color boxes so black is the foreground color and white is the background color.
3. In the Toolbar, select the Gradient tool.
4. In the Options bar, make sure the Gradient option displays a black to white gradient. If it does not, click that option to open the Gradient Editor, select the Foreground to Background gradient preset, and click OK.
   
   
   
   Tip: If you want to add multiple gradients to the same layer mask, select the Black to Transparent gradient preset in the Gradient Editor.
   
   
   
5. In the Options bar, select the Linear Gradient option. 
   
   
   
   Tip: You can use gradient shapes other than a Linear Gradient on a layer mask by selecting another gradient type, like a Radial Gradient, in the Options bar for the Gradient tool.
   
   
   
6. In the document window, click the area where you don’t want the adjustment applied and drag toward the area where you do want the adjustment applied, creating a black to white gradient on the layer mask.
   
   
   
   The white part of the gradient on the layer mask shows the adjustment at full strength on the underlying photo layer; the shades of gray on the layer mask gradually hide the adjustment; and the black on the layer mask hides the adjustment completely. This creates a gradual transition between the areas of the photo that are affected and not affected by this adjustment layer.
   
   
   
7. If you’re not happy with the result, repeat the preceding step, varying the length or direction of the line until you’re satisfied. You can also paint on the adjustment layer mask with the Brush and black, white, or gray paint to fine-tune the mask.

Save your work with layers

• Save the image in .PSD or .TIFF format to retain the layers and layer masks for future editing.