There are two different curves within the Tone Curve panel – the Parametric and the Point curve. While both curves can be used to make adjustments to the tonal values in an image, the Point Curve can also be used to change color values.
The Histogram displays a visual representation of all of the pixels values in an image, plotted from black on the left to white on the right; the height of the column shows how many pixels in the image have that value. The values are measured in 16 bit and are displayed as a percentage (between 0-100%). The Histogram can be an essential tool for managing the dynamic range of an image so that you can be sure you aren’t pushing important details in an image’s highlights or shadow areas to pure white or black without detail.
Here are eight Shortcuts for Working with Channels in Photoshop.