Color Correcting - the Levels Dialog

Posted on Monday, July 21, 2008:: 806 Views

Hopefully, if you're reading this, you've already read, or at least taken slightly more than a passing glance at our Histogram, Color Channels & Color Theory and Color Management Articles. If not, that's OK too. But having a stronger background in color, provided by the aforementioned articles, can only help your understanding of the topics covered in this article.

OK, on to the nitty gritty. Most image editors will have very similar color correction tools. The standard arsenal usually includes the following: Levels, Curves and Hue & Saturation. For the purpose of this article I am going to be covering Photoshop CS3's color correction tools, but Levels, Curves and Hue & Saturation generally all have the same anatomy no matter what your editor of choice is.

The Levels Dialog from Photoshop.	The Curves Dialog from Photoshop.	The Hue & Saturation Dialog from Photoshop.

Typically, you'll use just one of the above tools to correct an image, but there are cases where you might use a combination of two or even all three. The basic function of all three is to correct color/density issues and the saturation or desaturation of certain colors. When it comes to color correcting, Levels is just fine for many people. However, you might crave the absurd amount of control you can acheive with Curves. You'll find that it ultimately comes down to personal preference.

Keep in mind that a lot of color correcting  is a very subjective process. But there will always be images that even the "warm" people and the "cool" people can agree on, need some serious help. Sometimes, there may not be anything wrong with an image at all, you might just want to pump up the color or shift it in some dramatic and creative way. We'll take a look at all sorts of different scenarios.

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Dissecting Levels

Most of you have probably used Levels at some point, maybe even regularly. Whether you're alltogether new to Levels or a repeat user, knowing the mechanics behind one of the most widely used color correction tools in any image editer can greatly improve your correction results.   To summarize what Levels can do; you can fix color casts, improve contrast and "warm up" or "cool down" an image. When I say "warm up", I'm referring to adding yellow or red to give the image what can only be described as "warmth". The "cool" colors are in the blue/cyan range.   Did you know that when you make a Levels adjustment, you're actually adjusting the RGB histogram that is displayed inside Levels? Of course, you look at the image while making your adjustments, but the histogram is actually being altered. Now, as I've mentioned in the Histogram article, the histogram inside Levels won't update to reflect any changes, but you can have the histogram palette (in Photoshop) open to see your changes (maybe CS4 will fix this?).   *One advantage to working with Levels in Photoshop is that you can use it non-destructively as an adjustment layer.

 *For the sake of simplicity, we'll be dealing exclusively with RGB Levels in Photoshop CS3. Starting at the top of the dialog, when you color correct an image, you generally flip back and forth between the different color channels. With the RGB channel selected (which is just a composite of the three color channels, red, green and blue), you're affecting the brightness of the image. To target a specific color, you select a color channel from the channel drop down menu. For example: to lessen the blues in the midtones or highlights, you would select the blue channel and adjust the appropriate sliders. 

Moving on to the input sliders underneath the histogram. When working in the RGB channel, the slider on the left, the black point slider, controls the shadows, the middle slider controls the mid tones and the far right slider, the white point slider, controls the highlights. While adjusting the black and white point sliders, you can press and hold ALT (PC) or Option (Mac) to see a threshold view of the image which shows where the shadows and highlights clip.

 

While holding ALT (PC) or Option (Mac) and dragging the black point slider, the image will turn white, and where the specks of black appear, you have clipped shadows. This is handy when setting a new black point because you can see just how far you can push the blacks in your image.	The same applies to the white point slider, only, instead of the screen turning white, it turns black. As you've probably guessed, the white specks represent the clipped whites or highlights.

With the output sliders (underneath the input sliders) you can decrease the maximum black value and white value (while in the RGB channel). Default values are 0 for black and 255 for white. Moving the black output slider to the right makes the darkest black value lighter. Moving the white output slider to the left makes the lightest white value darker. Just think about the scale from 0 - 255. Full black is at 0. If you adjust the black output slider to 50 for example, you're darkest shadow values will be a dark gray. 50 is a lighter grayscale value than 0. If you set the white output to 200, your brightest whites will actually be a very light gray. 200 is a darker grayscale value than 255. Let's take a look at this with a black to white gradient.

		This is what our new histogram looks like. Notice how we have no more true blacks or whites after adjusting the output sliders so drastically? Our shadows and highlights start a good way in from the left and right sides of the histogram. Contrast would be shot if this was an actual image.
Here we have a black to white gradient. The left side has a value of 0, meaning it is full black, and the right side is full white at 255. Notice the histogram, it extends from the far left side to the far right.	When we set the black output slider to 50, the left side of the gradient becomes much lighter, since we're mapping maximum black to a value of 50 which is a dark gray. The full blown white on the right side is now a light gray now that we've mapped max white to a value of 200, which is a light gray. * Note: the histogram in Levels does not reflect this. See histogram above right.

There's an easy way to color correct using Levels without selecting any channels and without moving any sliders; the eyedroppers located just above the preview check box. To set a new black point, select the black point eyedropper and click in an area that you think should be black. Levels will automatically adjust the sliders in each channel. In fact, after setting a new black point, check out the sliders in each channel, they should be in a new position. The gray point and white point sliders work in the same way. Select each one and click in an area that should be neutral gray and white respectively.

And if all else fails, there's always the dreaded Auto button. Be wary of this button as it won't always (and usually doesn't) yield good results. However, there might come a time when it does a bang up job. Sometimes Auto provides a good starting point. If Auto does make a mess of things (as it's been known to do) simply press and hold ALT (PC) or Option (Mac) and the Cancel button will turn into a Reset button.

 You may have noticed the Load and Save options. If you find you are constantly using the same correction, you can save it and load it later to use on all your images.

Before moving on, let's take a look at how the input and output sliders work in the color channels. In order to correct a color issue (rather than a brightness or contrast issue), you need to select one of the color channels in the channel drop down menu at the top of the dialog. The best way to understand how the input sliders work is to actually use them and note the changes that occur. For the example below I've chosen to use the red channel. 

When woking on a color channel, the graph represented by the histogram is different from the grayscale ramp of the RGB histogram. Instead of a gradation from 0 (black) to 255 (white), you have a graph of saturation values with 0 on the left and full saturation (255) on the right.
*The output gradient in Levels doesn't look  any different in the color channels, but it is in reality the gradation you see to the right.	In the red channel, 0 saturation is cyan because cyan is the opposite of red. Think of the left slider in the red channel  (the black point slider in RGB) as the "cyan" point and the far right slider as the "red" point.	The actual red histogram (shown here from the histogram palette) only dislpays a red bar graph, but the red values on the far left of the graph are actually cyan.

 

Moving the "cyan" point slider to the right in the red channel compresses the histogram to the right. According to the new value, cyan now starts at 100. This results in less red and more cyan in the shadows. See how the cyan in the gradient above is pushed to the right?	Moving the mid point slider to the right in the red channel decreases the red in the midtones. In other words, the mids will contain more cyan. To add red to the mids, you would move the slider to the left.	Moving the "red" point slider to the left in the red channel compresses the histogram to the left. Full red now starts at 100 versus 255. This results in very red highlights.

Like the input sliders, the output sliders behave differently inside the color channels.

	Notice how there is no more cyan in the shadows (the left side of the histogram). The range of red values is much shorter now with no truly cyan values.
Moving the "cyan" output slider to the right is going to add red to the shadows and lighten them up a bit. You're basically defining cyan as being more red. Instead of being 0 (with no red at all), cyan is now set to 100, which has much more red in it. Think of it this way; instead of the scale going from 0 to 255, or full cyan to full red, it now goes from 100 to 255, or not quite cyan to full red.

	Notice our red highlight values disappeared. We've now compressed the histogram in the opposite direction. Again, the range of red values is much shorter with no real fully saturated, bright reds.
If we move the "red" output slider to the left, we eliminate red in the highlights. This is going to add cyan to the highlights and also darken them up a bit. Our scale now goes from 0 (cyan) to 175 (not full red).

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Analyzing A Red Channel Adjustment In An Image

Let's take a look at how the input and output sliders in the red channel in Levels affect our oranges image. In the images below, the right side is being affected by a Levels adjustment in the red channel whereas the left side is the original, untouched image.

By moving the "cyan" point input slider to the right, I've made the shadows slightly darker and very cyan. Since the adjustment was so extreme, cyan has even spilled into the mids.	Here I've made a much more subtle mid tones adjustment. By dragging the slider to the right, I've made the mid tones "cooler" by adding cyan.	Another extreme example. In this case I adjusted the "red" point slider to the left until the input value read 175. As you can see, the highlights are bright red. Many of the lightest red areas are now blown out. Because of the extreme adjustment, red has spilled into the mids as well.
Here I've set the "cyan" output slider to 100, brightening the shadows and turning them red.	As you can see, by moving the "red" output slider to the left, I've darkened the highlights and made them extremely cyan.

 

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Hopefully you remember a little of your color light theory. If not, then the following diagrams will help when adjusting each color channel.

When working in the red channel in Levels, the far left sliders will control cyan and the far right sliders will control red. The middle slider will control the amount of red in the mid tones; less red in one direction (meaning more cyan), or more red in the opposite direction.	When working in the green channel in Levels, the far left sliders will control magenta and the far right sliders will control green. The middle slider will control the amount of green in the mid tones; less green in one direction (meaning more magenta), or more green in the opposite direction.	When working in the blue channel in Levels, the far left sliders will control yellow and the far right sliders will control blue. The middle slider will control the amount of blue in the mid tones; less blue in one direction (meaning more yellow), or more red blue the opposite direction.

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