How to work with Curves
To work with Curves
Select the Choose Color tab from the Menu Bar and choose Adjust in the Menu. After that click Curves.
Use the Curves Adjustment tab to map any brightness value to a new value. This tab gives you more flexibility than the Levels tab because you can edit any value from (0 to 255) (black to white) scale.
The graph charts the relationship between the input and output levels. At the lower left of the chart, both values are 0 (black). At the upper right, both values are 255 (white).
Select a channel to edit from Channel drop-down box. You can edit all the R/G/B values simultaneously or select a single channel.