Adjustment Layers
An adjustment layer applies colour and tonal adjustments to your image without permanently changing pixel values. For example, rather than making a Levels adjustment directly on your image, you can create a Levels adjustment layer. The colour and tone adjustments are stored in the adjustment layer and apply to all the layers below it. You could always go back to this adjustment layer and:
- reconfigure its settings.
- adjust the adjustment layer’s opacity and blending mode.
- group to apply the adjustment to specific layers.
- turn the adjustment layer’s visibility on and off to apply their effect or to preview the effect.
To create an adjustment layer, select the middle icon 
at bottom of the layers palette that looks like a circle (half black, half white). If you hover on the icon, a tool tip will pop up saying: “To create a new fill or adjustment layer”. If you click on the icon, a context menu will pop up allowing you to select which adjustment layer you would like to create.
If you have adjustment layers in an image, you will not be able to save it as a Jpeg but instead as a Photoshop file (.psd extension). You will need to flatten your image in order to save it in Jpeg format.
Further Resources
- Helen Bradley, 2006, Working Smarter with Photoshop’s Adjustments Layers, Photoshop CS2 Speed Clinic, The Automating Photoshop to Get Twice the Work Done in Half the Time, Peachpit Press