Curves is the ultimate tool in Photoshop for changing the tonality or adding or removing a colour cast.
A curves adjustment gives more control than using levels, or any other tonality tool for that matter.
Curves: Basic Principles
If you move the baseline upwards the image gets brighter. If you move the baseline downwards the image gets darker.
The area of the image which is made darker or brighter is dependent on the histogram underneath the baseline.
The shadows are on the left. The highlights are on the right.
Black and White Points
If you drag the black point slider to the right you clip the shadows.
If drag the
If you hold down the Alt, or Option key whilst dragging these sliders you will see any clipping, as coloured areas.
Curves Adjustment Layer Diagram
What does “Auto” do?
The Auto settings is an “Enhance Brightness and Contrast”, which is content aware. It does not touch the individual colour channels. So, it does not remove a colour cast.
Quickest way of using Curves in Photoshop
Holding Alt (Windows), Option (Mac OS) press the “Auto”. This will invoke the “Auto Color Correction Options”.
Ticking “Snap Neutral Midtones” ensures the first three a
Go through the four Algorithms to find which one you like, then press “OK”
Now you can use the “Targeted Adjustment Tool” on the IMAGE to brighten or darken specific areas.
You could even use the Targeted Adjustment Tool in the colour channels.