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Levels and Level Adjustments

Introduction:
The levels adjustment in Photoshop is used to adjust specific brightness values for shadows, midtones, and highlights in each of the three color channels. (3 in RGB, 4 in CMYK) If you don't know what a histogram is, I would suggest reading this first prior to proceeding with the rest of this article.
 
Levels:
This is the basis of the Levels Tool. You can access the Levels tool by clicking on Image --> Adjustments --> Levels or ctrl + L
 
Levels Dialog
 
Lets take a look at what each of the components of this dialog and see what they do.
 
Channel:
In this box you specify which color channel you would like to edit. You can edit them individually or work on the composite if you so desire.
 
Input Levels:
This slider (or input boxes) let you change the contrast of the image. This is accomplished by using either the dark or white triangle adjustments. When you mode the black triangle, you create a new black point. Everything to the left of this point will be black. The reverse is true for the white triangle adjustment. You can create a new white point by moving the white triangle towards the left. Everything beyond this point to the right will become white. Be careful not to overdo it. Adjusting either of these points has no effect on the other giving you much more control than the Brightness & Contrast tool.

To see the effect that your slider adjustments are having on the image it's a good idea is to hold the Alt key while adjusting sliders. This shows you what is being clipped as you make an adjustment. Sometimes you need to make a very small change and just looking at the image itself is not going to be enough, and using this method you will be able to see exactly how what pixels the your changes are affecting.

The small gray triangle in the middle is used to adjust the midtones independently of black and white. Slide it to the left and the midtones will lighten or slide it to the right to darken them.
 
Output Levels:
This option is used to limit the brightness range of the image. For example if you set the darkness (left triangle) to 30 then no color will be darker than a brightness value of 30. The same applies to the brightness (right triangle) to 50, then no color will be brighter than a brightness value of 50.
 
Step by Step:
So, to fix an image, follow these steps.
  1. Open the levels dialog and select the red channel.
  2. Drag the black and white input triangles to the beginning of the histogram, that is the point at which the histogram begins.
  3. If the image seems a bit dark or bright, now is the time to adjust the middle gray triangle to fix it.
  4. Repeat steps 1 to 3 for the other two channels.
  5. Once you have your colors corrected, have a look at the composite histogram and see if you need to adjust it any further.
  6. Click ok.
If done carefully, this procedure is generally the best method for fixing exposure problems for your images. It is a bit more time consuming than some of the other adjustment methods available, but produces much better results with relatively little effort.

Cheers

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