The Adjustment Layers in Photoshop are a group of a super useful, non-destructive image editing tools that add color and tonal adjustments to your image without permanently changing its pixels.
With the adjustment layers, you can edit and discard your adjustments or restore your original image at any time. This will make your workflow in Photoshop more flexible and efficient, and is an absolute must-know.
There are 3 different ways to add an adjustment layer:
Layers Panel - Click the New Fill or Adjustment Layer icon at the bottom of the Layers panel, and then choose an adjustment layer from the list. (Note: the first three items in the list—Solid Color, Gradient and Pattern—are fill layers, not adjustment layers. Choosing one will simply fill the document with either a solid color, a gradient or a pattern.)
Layer Menu - In the menu bar go to: Layer > New Adjustment Layer
In the Adjustments Window, click on the icon next to the layer you'll want to create.
Brightness/Contrast: Brighten up an image and add contrast.
Levels: Make precise exposure adjustments, color corrections, or add stylistic color grading.
Curves: Similar to Levels, but with added precision.
Exposure: Make quick adjustments to the overall exposure of an image.
Vibrance: Make colors pop without affecting skin tones.
Hue/Saturation: Target and change any colors in a photo.
Color Balance: Make subtle color shifts for color correction or color grading.
Black & White: Instantly convert an image to black and white while adjusting the tonal values of each color channel.
Photo Filter: Add warming or cooling filters for quick color correction.
Channel Mixer: Convert an image into greyscale, sepia tone, or other color tints.
Color Lookup: Add beautiful color to your photos by creating and saving LUTs.
Invert: Creates a photo negative effect.
Posterize: Reduces the the number of brightness values in an image for a graphic effect.
Threshold: Great for matching color and contrast when compositing images together.
Gradient Map: Color grade photos with a custom gradient, adding specific colors into the highlights, midtones, and shadows.
Selective Color: Shift cyans, magentas, yellows, and blacks to change the appearance of those colors in an image.
Unlike standard image adjustments where the settings open in a separate dialog box, the settings for adjustment layers appear in Photoshop's Properties panel.
Depending on which type of adjustment layer you created, you'll see sliders, graphs or settings to change.
By default, the whole image is selected so any adjustments will affect the image as a whole.
You can turn an adjustment layer on and off to compare its effect with the original image by clicking the layer's visibility icon (the eyeball) in the Layers panel:
A big advantage with adjustment layers is that the initial settings we choose are not permanent. We can go back to the Properties panel and edit the settings at any time.
If a different layer is active in the Layers panel, simply click on the adjustment layer's thumbnail to reselect it:
Adjustment Layers are great because they’re an easy way to make edits non-destructively. They also function just like any other Layer, so you can use Layer Masks, Blending Modes, and Layer Styles to make sure they’re only affecting the areas you want them to.
Adjustment layers automatically have masks linked to them. The masks are all white by default, meaning that you can see the effects of the adjustment layer on the whole image. You can paint black on the mask to block different areas of the mask from showing the adjustments.
Command-I inverts the mask to be all black (hiding all the adjustments). Then you can paint white on the mask on specific parts of your photo to show the adjustments just on that part.