By working on a duplicate background layer, you can turn it on and off to check on progress in the editing process. If things go wrong, use the History palette to start over, by selecting the Background layer. Or you can use the Revert command to go back to the last saved version. If you have saved frequently while editing this can be a problem. It is always best to work on a copy of the original file.
Adjust Tonal Range
Levels - The image consists of 256 shades of gray in each of the colors. This is called the Tonal Range. A lesser the amount of gray, the more vivid the color. The greater the amount of gray, the more subdued the color.
In a monotone image there is no color, only shades of gray.
The Levels command allows corrections to be made when the camera does not distribute the Tonal Range properly.
The Histogram
The Levels dialog box displays a Histogram that represents the distribution of the gray pixels.
A Histogram with a rounded mountain that extends to each side of the box, can be considered an average or normal distribution of the Tonal Range.
A Histogram with steep spikes at each end and relative flat in the center, is likely to be a high contrast image.
A Histogram that is flat from one side to the other would be considered a low contrast image.
If the mountain is mostly to the left, is a Low Key Image, mostly darker tones or black.
If the mountain is mostly to the right, is a HIgh Key image, mostly light tones or white.
Adjusting the Tonal Range
Below the Histogram display is a slider with 3 triangles.
On the left is the black triangle and adjusts the darker end of the spectrum, or the shadow region.
In the the middle is a gray triangle and adjusts the middle range of the gray scale.
On the right is the white triangle and adjusts the lighter end of the spectrum, or the highlight region.
The procedure is the move the black and white triangles to where the slope of the mountain begins to rise. The first effort is not critical.
Then move the gray triangle until you have a pleasing image.
After things are generally in place you can make minor adjustments to the triangles until you are pleased with the results.
Adjusting the Tonal Range with the slider triangles only adjusts the gray component (RGB) of the image. You can adjust each of the Red, Green, and Blue colors individually. The Auto button does this automatically. However I have found the Auto function to be problematic and find I can do it easier and better with the Color Variations command.
On the rare occasions when an image has to much contrast. You can adjust the sliders in Output Levels to reduce the level of contrast.
By using a Levels adjustment layer, you are able to make all of your corrections without affecting original image. There is no degradation from recalculation after a levels command is applied. As a result you are able to make changes to the Levels adjustment layer at anytime. In case some other command has upset things.