Adjustment Layer

If you want to correct the colours in multiple layers, you have to create a special kind of layer called an adjustment layer. Adjustment layers are layers that contain colour correction information. The layer applies its corrections to all layers beneath it, without affecting any layers above.
You can create an adjustment layer in one of two ways:
1. Choose Layer>New Adjustment Layer. This displays a submenu of colour adjustment commands.
2. Click the half black/half white circle at the bottom of the Layers palette.

If you just want your correction to be applied to one layer only you can ‘Clip it’ to the layer below it: hold down Alt , position the mouse over the line in the Layers palette dividing the base layer and the Adjustment Layer (the pointer changes to two overlapping circles ), and click.

When editing the settings for an adjustment layer, be sure to double-click the first icon that precedes the layer name.

The advantages of layer-based corrections
1. Forever editable:
As long as the adjustment layer remains stored in the native Photoshop PSD
you can edit the colour correction without damaging the underlying pixels. Unlike standard colour corrections, which alter selected pixels directly, adjustment layers have no permanent effect on the pixels.
2. Versatile layer masking:
An adjustment layer covers the entire image. You modify the affected area by painting inside the layer. Paint with black to remove the correction from an area, use white to paint the correction back in. You can edit it well into the future without any adverse side effects.
3. Reorder your corrections:
As with any layers, you can shuffle adjustment layers up and down in stacking order. For example, if you decide you don’t want the correction to affect a specific layer, just drag the adjustment layer to a level in the Layers palette below the layer you want to exclude.
4. Fade corrections:
You can fade a standard colour correction right after you apply it using Edit>Fade. But you can fade a correction applied with an adjustment layer any old time. Just change the blend mode and Opacity settings in the Layers palette.
5. Change one adjustment to another:
After applying one kind of adjustment layer, you can convert it to another kind of adjustment layer.

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