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The Levels control allows you to set black and white points for your image. This changes the tonal range of your image, giving it full contrast.

Look at the following sample image...

Simple visual inspection reveals that the image lacks contrast, and the dynamic range of the image is compressed when you inspect the histogram in the Levels dialogue. Note that the histogram leans mostly to the left and is not spread evenly across the x-axis indicating compromised tonal range and, in some cases, underexposure. The image is, very simply, dull.


Now look at the image after a Levels adjustment...

Notice how the contrast has been improved and how the image has an improved overall appearance. This was accomplished by dragging the white slider to the left and the black slider to the right in the levels dialogue. In this way we increase the tonal range of the image. In effect, we have shortened the x-axis of the histogram.

This adjustment can be applied directly to the image's pixels by choosing the Levels tool from the Image/Adjustments menu. However, we want to use an adjustment layer, as discussed previously.

How do we do this with layers? View the Levels video and then return here for a text explanation.

Levels Video

By holding down the ALT key and dragging the sliders you see a threshold screen that displays red as the highlights are beginning to be blown out in your image, and black as the dark areas begin to lose detail in your image. We stop moving the slider just as this begins to happen. This maximizes the tonal range of your image and increases contrast.

There are no hard and fast rules that can be given that tell us when to stop dragging the sliders. It is a somewhat subjective decision. You must look at the result to see if it is pleasing to you. If not, simply make a change by double-clicking the the circle that appears to the left in the Levels layer in the Layer Palette. The Levels dialogue will reappear so you can make your changes. 


Because you have used an adjustment layer to perform this task, you can come back to the layer, open the adjustment, and make changes any time you wish - even after other adjustments have been made to the image. Best of all, you have not changed the pixels in the image at all. No image degradation has taken place. We can be assured that, at the end of our editing session, we will have an image of the highest quality.



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