NOTE: This article is for older versions of Photoshop
(prior to CS6/CC), but the fundamentals still apply.
The Levels Adjustment Layer is used for contrast control and color correction. It’s an extremely versatile tool. If I only had one function available to me in Photoshop, this would be the one. In a well-exposed image, it’s entirely possible that with a few simple adjustments in levels, your image will be ready to print.
Where to Find:
You can click on the “New Adjustment Layer” button in the Layers Palette (the Yin-Yang” button), and choose Levels from the drop-down list. You can also go to the “Layers” menu ->”New Adjustment Layer” and choose it from that menu.
Note: By the way, there are two places you DO NOT want to open an Adjustment Layer: The “Image” menu -> “Adjustments” and the speed keys that are shown in that menu. The reason? If you choose either one of these locations, a separate layer is not created, which defeats the whole purpose of layers.
Definition of Contrast:
As you’ll recall, the definition of Contrast is the difference in brightness between the light and dark areas of an image. As our eyes generally like “snappy” (higher contrast) prints, you are trying to achieve the maximum “snap” or high contrast without blocking up shadow areas, while at the same time retaining detail in the highlight areas.
The Histogram is a graph that represents the total range of an image, as measured in pixels. It basically tells you how your image is exposed. On the left side of your histogram, 0 represents all “dark” pixels, and on the right side of the histogram, 255 represents all “light” pixels. The higher the graph for a particular value, the more information there is at that level.
So, generally speaking, an image that is under exposed will have more values on the left side of the graph, an image that is over exposed will have more values on the right side of the graph, and an image that is properly exposed will have values evenly distributed throughout the histogram.
Please note that I said “generally speaking” in the above sentence — there are always exceptions to the rule. For instance, a scene of snow may print well, but its histogram will be weighted with more “light” pixels, whereas a night scene may also print well, but its histogram will be weighted with more “dark” pixels.
In my opinion, it’s easy to place too much emphasis on what a histogram shows you. It is important, to be sure, but you should use it as a guide, and not let it become your “gospel”. How your final image looks should be the deciding factor in what works and in what doesn’t. No one looks at a print and says, “Oh, that’s a great print, but the histogram looks terrible!”
A Color Channel stores information about color elements in the image. You will note that in the Drop-Down menu, you have four choices: RGB, Red, Green, and Blue. Each one of these is a color channel, with RGB being a composite of all three colors.
White Point /Black Point and Clipping:
Setting the White Point and Black Point allows you to set the brightest parts of your image as close to pure white as possible and the darkest parts as close to pure black as possible.
Adjust the Black Point in an image by moving the “Shadows” Slider to the right, and the White Point by moving the “Highlights” Slider to the left.
To see a Clipping Warning, hold down the “Option” key in Mac (“Alt” key in Windows) as you move the “Shadows” and “Highlights” Sliders. Adjust both until clipping just starts to occur.
Generally speaking, clipping is less critical in the Shadow areas, which is why I like to start at the value “7” for my “Shadows” adjustment value (see “To Adjust Contrast” below).
Note: When using the Clipping Warning, it should be used mainly as a guide. What your eyes see is still the best judge of how your image should look.
To Adjust Contrast:
As you can see in the above diagram, you can control the values of Shadows, Midtones, and Highlights. Most of the time, I generally type in the following values as a starting point: 7, .97, 255 while in the RGB Color Channel. By the way, you can jump between these boxes by pressing the “Tab” key.
~ Move the “Shadows” Slider to the right, or input a higher numerical value, to increase contrast in the shadow areas. This also sets your Black Point.
~ Move the “Midtones” Slider to the left (or increase the numerical value) to decrease the contrast in the midtones, and to the right (or decrease the numerical value) to increase the contrast in the midtones.
~ Move the “Highlights” slider to the left to increase contrast in the highlight areas. Be very careful with this one — you don’t want to lose any highlight detail. You can avoid this by not dragging the slider so far to the left that it starts to intersect with increasing “light” values in your histogram, and checking your Clipping Warning as described above. This sets your White Point.
~ Click on the Drop-Down Menu, and pick the Color Channel that you desire to add or subtract.
~ Use the Sliders or input the numerical value to make your desired color correction.
Example: If your print needs more Red, go to the “Red” Channel in the Drop-Down Menu, then move the “Highlights” Slider to the left to increase the red cast. Conversely, if you desire more Cyan (The opposite of Red), you would move the “Shadows” slider to the right to increase the Cyan cast.
Here are two “real-life” examples on how to use Levels, followed by commentary as to what was done and why….
This example is a correction for Contrast only:
As you can see, the original “Uncorrected” image on the left is flat, a bit dark and lifeless. The histogram reflects this: there is very little information in the dark areas and light areas of the graph – just lots of midtones.
The “Corrected” version on the right has more pop to it both in color and contrast, resulting in a much more pleasing image. To achieve this, the “Shadows” slider in the RGB Color Channel is moved to the right to a numerical value of 37 to increase the saturation of the green in particular, the “Midtones” slider is moved to the right .95 to increase punch in the midtones, and the “Highlights” slider is moved to the left to 207 to lighten up the overall print.
This example is a correction for Color only:
The “Uncorrected” image on the top left has a cyan/blue cast to it.
To correct (see “Corrected” image on the top right), the “Highlights” slider in the Red Channel is moved to the left to increase the red bias to offset the cyan cast. In addition, both the “Shadows” slider and the “Midtones” slider in the Blue Channel are moved to the right to increase the yellow (the opposite of blue) bias, offsetting the blue cast.
By the way, it is not unusual (in fact, it is the norm!) to have to correct with more than one Color Channel. In this case, adding red was not enough. The image needed an additional amount of yellow to get the proper color balance.
Until the next column, have fun and stay safe!
by John Watts, Watts Digital Imaging
All text & photos / screen shots: © 2010 John Watts, Watts Digital Imaging. All rights reserved.