Photoshop Tutorials - Levels
In the Levels window you will see the same diagram as in the Histogram window. Check the Preview box at the bottom of the window so that the changes would be instantly displayed in the image window. There are three sliders at the bottom of the histogram that display where the black, white and gray scale colors are located.
Move the black slider to the right and the image will become darker. Set the black color slider so that the dark area of the histogram that does not contain pixels would be outside the image tone range.
Do the same thing to the right slider - move it to the left until the light area of the histogram that does not contain pixels would be outside the image tone range. This will make the shadows look deeper, some image elements will become bright and the image details became more notable.
When the Shadows and Highlights values are set you will note that the image is too dark and therefore some details are missing. You need to correct the gray scale. Move the Gray Scale slider towards the Shadows area. The neutral grey value will become darker and the image will become brighter.
The Input Levels section of the Levels window displays the numerical value of Shadows, Highlights and Gray Scale. These values automatically change as you make corrections manually or you can make corrections entering your values into the respective sections.
Click the Ok button to close the window and complete correction. Open the Levels window again. As you can see the Histogram looks different now. Grayscale tones are spread along the interval, but there are empty spaces between them - this is the retribution for stretching the tone range. Some tones may even be missing. This will not be notable.
Using droppers with different fill (black, white or grey) you can determine the image areas that you want to be the darkest (black dropper), lightest (white dropper) or gray scale (grey dropper). Grey dropper is used during the correction of the color images. The dropper tools in the Levels window are easy to use. Select the black dropper and click the tone that you want to be the darkest in the image and all of the tones that are darker will disappear. White dropper works the same way.
If you plant to print the selected image, you need to take into consideration the drawbacks of the color output of the printing machine. Printers have problems with output of the very bright and very dark colors. Therefore, the amount of tones in Shadows and Highlights will decrease. In order to have these tones displayed you need to move the shadows to the lighter areas and move highlights to the darker tones. This operation is a contraction of the tone range, or cutting down the output levels.
You can cut down the output tone levels by moving the slider in the Output Levels section of the Levels window. After the tone range will be contracted, the image will become grey. To compensate the loss of image quality you need to edit the input levels of white and black, increasing the contrast. If Shadows and Highlights have few pixels these tones will not be distinguishing on the print. It is wise to remove them stretching the Input range, as this will increase the contrast that was lost contracting the output range. Deleting the darkest and lightest tones is achieved by cutting down the edges of the range that have pixels.
Here is what each slider is used for, in Input and Output Levels section.
Input Levels (increases the contrast):
Black slider - darkens the dark pixels
White slider - brightens the bright pixels
Grey slider - darkens/brightens up the midtones.
Output Levels (decreases the contrast):
Black slider - brightens the dark pixels
White slider - darkens the bright pixels.
To restore the image without closing the window you can press Alt and the Cancel button in the window will turn into Reset button. Click Reset to return to initial image settings. Such hidden Reset button exists in many Photoshop windows.
The levels correction allows to change the basic tone characteristics, set the tone range and change the location of the neutral point (change the image color spectrum).
In the Levels window you can perform the simplest operations to correct the contrast and luminosity of the image in the mid tones (using the color spectrum) and in general (editing the Output Levels). Playing with the Output Levels you can set the brightest and darkest tones of the image for the print. It is useful to save the image editing parameters, especially when you need to edit a series of images in the same way.
You can save the Levels settings so that you could make similar changes to the set of images. You can save all of the window settings by clicking the Save button. You can activate such settings for the image by pressing Load button. Click the Options button to set the luminosity values that you want to cut down. However, automatic tonal correction is not very effective, as you will see trying it.