Getting Started With Histograms For Color Correction

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Getting Started With Histograms For Color Correction

The original and color corrected images are shown.
Understanding how to read and use a histogram is important.

Talk to any imaging professional and they will tell you that no image is ever perfectly exposed. You will always need to correct for the inevitable dark areas and light areas. There is a faint color over the image called a color cast. Maybe the colors just don’t “look right”. After all, any decisions made about color are subjective, not objective. It is how you “see” the colors, not how others “see” them.

Still, there is no image taken by your smartphone, a point and shoot digital camera or a high-end DSLR that can’t benefit from a few “tweaks”.  The question is: “Where to start?”

Before we get into answering that question let’s step back a bit and understand that color correction requires you to be a bit detached from the image. By that I mean, instead of concentrating on the subject of the image, you need to start looking at the image from a technical point of view. It starts by looking at the image and asking a few questions:

  • Is it too dark?
  • Is it too light?
  • Is there a predominant color?

Answering those questions is the first step in determining your correction strategy and the odds are almost 100%, as you shall see, that your strategy is wrong.

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How To Read A Histogram

Histograms in Photoshop, Apple Photos, Affinity Designer, Creative Toolkit and Photoshop Elelements are shown.
Histograms are a core feature of practically every imaging application.

Practically every imaging application on the market will contain a Histogram. In Photoshop you can see a Histogram by selecting Image>Adjustment>Levels. In Lightroom, it is there in Develop mode and in such applications as Luminar, Aurora HDR, Affinity Photo and others, it is always visible.

 A Histogram is a graph that shows you tonal distribution in the image. If the graph is primarily over on the left, the image is dark. If it sports a graph that has a bump in the middle with gentle fall off to the right and left, the image is somewhat balanced. If the graph is primarily over to the right, the image is too light.

The sliders you see at the bottom determine where the shadows, midtones and highlights are set. Notice they are placed at the left, middle and end points of the graph. They represent the full range of grey scale colors. The black is 0, the midpoint is 128 and the white is 255. If the graph is clumped between the the black and grey sliders, the image is dark and is said the be “Low Key”. If the image has that bump over the grey slider is said to be “Mid Key” and if clumped between the grey slider and the white slider the image is bright and said to be “High Key.”

This is where you need to determine your correction strategy. In a Low Key image the tonal values are distributed between 0 and 128 and the strategy will be brighten the image by increasing the range toward the white point. In a High Key image the the tonal values are clumped between 128 and 255 and the strategy will be to darken the image by shifting the range toward the black point.

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How To Use A Histogram

The Photoshop levels panel containing a histogram is shown.
Histograms display tonal values and set your correction strategy.

By moving the sliders in the Histogram, you can start the color correction process. Before you start, look at the graph. If you have a spike over the white or black sliders, those are pixels that are either pure black or pure white and they contain no color data. If there is a  vertical “flat line” rising from the graph to the white or the black point, those pixels have minimal color data.  In that case, moving the black and the white sliders to the start of the flat line will bring out detail.

You may also notice the numbers associated with the slider change. In the above image, for example, the number for the Black slider is 20. What does that mean?

What it means is that you have “clipped” the black value. In simple terms, all pixels with a value between 0 and 20 now have a value of zero. This explains why the image darkened when you moved the slider. It means exactly the same thing when you move the white slider.

If you move the mid point slider you are adjusting the mid tones. Move it towards the black point and the image brightens because the dark tonal range is being reduced. The same goes for moving it toward the white point.

Another indicator that highlights or shadow have been clipped is the presence of a tall vertical spike in a histogram and is commonly found in the highlights. You may l see black lines in the graph. These indicate areas where the pixels have no color value.

Another common Histogram feature is those three eyedroppers you see in Photoshop. Their purpose is to let you use the image to set the black and the white points. If you select the black eyedropper, click on a dark area and you have set the black point. Same goes for the white eyedropper. Just be aware that choosing a white or black point can be dangerous.

In the above image a logical white point would be the very bright area in the clouds. This would be a mistake because those bright pixels are shown in that spike over the white are specular white.

If you are a Photoshop user there is an even easier way of setting the Black and White points using the sliders.  If you hold down the Option/Alt key when moving a slider, the pixels being clipped will appear. For example, if you hold down the Option/Alt key when moving the black slider, the screen will be pure white. As you move the slider to the right  the pixels being clipped will start to appear. The color is the pixels in the color channel that are being clipped. With the white slider, the screen starts out black and as you move the slider to the right the pixels being clipped will appear.

What if you aren’t happy with the result? If you don’t accept the change, just press the Option/Alt key and the Cancel but changes to Reset.

One final note:  In Photoshop there are two places to access the Levels. The first method is to do what I have been doing throughout this section and use the levels command from the Layers menu. This is not exactly regarded as a best practice because it is destructive. By that, I mean, once you click OK those changes are “baked into” the image. The only way of changing it is to revert the image (File>revert) and starting over.

The non-destructive method is to use a Levels Adjustment Layer. By going this route you can always return to the Levels and make changes.

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How to Identify and Remove a Color Cast

The orginal image and the final image using curves and a histogram to remove a colour cast.
A Curves Adjustment Layer and the histogram are how you find and correct a colour cast.

Color casts are sneaky because they tend to hide in the highlights of an image. A perfect example of that is clouds that look bluish or that orange color you get when an image is taken indoors. There are a couple of hundred ways of finding a color cast and a few thousand ways of removing it. Let’s stick with one that uses our friend the Histogram.

Color casts always involve a predominant color in the highlights or ,occasionally, the Midtones of an image. In the above image you may think there isn’t a color cast but, in actual fact, it is there. If you open the Info palette and run the pointer over the highlights and Midtones of an image pay attention to the changing RGB values in the Info palette. If one color seems to be consistently larger than the other two as you move the cursor, you have found your color cast.

An even easier method is to use the Histogram to find the color cast. The above image has been opened in Photoshop and the Histogram panel is opened by selecting Window>Histogram.  

In the case of the above image, the color cast is Red. The Green and Blue channels don’t run the width of the Histogram. From this you can gather a color cast is the red color channel closest to the whites and/or blacks.

To fix it, add a Curves Adjustment Layer.

When the Curves appear choose the Blue and the Green channels and move their sliders to the start of the graph on either side. If you are looking for precision, don’t drag. Click on the point in the curve and press the Right or Left arrow keys to move the selected point. As you move the sliders, the Histogram for that channel will change as well. To finish up, you might want to make some minor adjustments on the layer causing the color cast. The point here is to have all three histograms somewhat in balance.

As I pointed out earlier, a number of imaging apps that I have covered on this site give you the ability to use a Histogram for color correction. Two that I have written about- Affinity Photo and On1 – come at the use of a histogram from slightly different directions than the Adobe applications. Let’s start with Affinity Photo.

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How To Use An Affinity Photo Histogram

The Affinity Photo Histogram and Curves panel are shown.
Affinity Photo from Serif has a rather easy to use Curves panel.

Affinity Photo’s Histogram is treated like a separate panel. If you don’t see the panel , it can be found in View>Studio> Histogram. One feature of the Histogram is you can roll your cursor over the graph and you will be shown the pixel count at the color level where the cursor is placed. This neat little feature is available through the Advanced option in the panel’s pop down.

You can also view the individual channels in color using the pop down to identify a color cast and to undertake correction.

In the above image there is a serious blue color cast. To remove it, click the Curves strip in the Adjustment panel and adjust the Highlight curve for the Red and Green and move the highlight handle to left. As you do both the curve’s histogram and the Histogram panel itself will adjust. As well, the color handles in the Curves histogram will also be visible. This makes fine adjustments an absolute breeze to accomplish.

Let’s wrap this up with ON1.

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How to use the ON1 Histogram

The ON1 Photo's Histogram and colour correction panels are shown.
The ON1 histogram is the hub of this imaging applications editing and effects features.

The ON1 Histogram is always available and can be turned on and off by selecting Histogram in the panels.

Though there are no curves in ON1, the Histogram adjusts to every move of any slider in a filter or Effect. For example, if you are in Enhance you can remove a blue color cast by moving the temperature slider in the Color area away from blue and slightly reducing the Vibrance. You can also adjust the sliders in the Tone area and as you move them, the histogram  also changes. The important ones are the Whites and Blacks sliders. They have essentially the same effect as moving Photoshop’s White and Black sliders in Levels or Curves.

To see a Before and After view of these changes, click the Preview button at the bottom of the image.

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Your Citation
Green, Tom. "Getting Started With Histograms For Color Correction." ThoughtCo, Nov. 28, 2016, Green, Tom. (2016, November 28). Getting Started With Histograms For Color Correction. Retrieved from Green, Tom. "Getting Started With Histograms For Color Correction." ThoughtCo. (accessed November 19, 2017).