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Transcript for Highlight and Shadow Values by Channel

Video -- Part I

Narration by Ron Bigelow

www.ronbigelow.com

Photoshop CS4 Used in this Tutorial

Hello, I am Ron Bigelow, and I would like to welcome you to my video on setting the Highlight and Shadow Values by Channel. Setting the highlight and shadow values is a very common procedure when editing images. However, most photographers tend to set these values using the RGB channel. In this video series, we are going to first look at setting the highlight and shadow values in this manner. Then, in Part II of this series, we will look at a more sophisticated method of setting the highlight and shadow values by using the individual channels. This method can often produce superior results.

Before we get started with Photoshop, we should first define what we mean when we say shadow values or highlight values. The shadow values are the three color values (i.e., red, green, and blue) of the darkest point in an image where detail still exists. Any points darker than the shadow values have no image detail and show as solid black. The highlight values are the three color values of the lightest point in an image where detail still exists. Any points lighter than the highlight values have no image detail and show as solid white.

So, let’s get started with this technique. Here we have an image of a river scene. Let’s start the editing by launching the histogram. Now, histograms display tonal information, if we are looking at the RGB channel, as we are here, or color information if we are looking at the individual channel histograms. There are a total of 256 tonal or color values in a histogram. The darkest tone or color is assigned a value of 0. The lightest tone or color is assigned a value of 255. The horizontal axis displays these tones or colors from the darkest on the left to the lightest on the right. The vertical axis shows the number of pixels that have each tone or color.

Now, this histogram shows a problem with the image. The area to the left of the histogram has few, if any, pixels. Since this end of the histogram displays the darker pixels, the lack of pixels at this end shows that there are few, if any, blacks or very dark pixels in this image. The area to the right of the histogram also has few, if any, pixels. Since this end of the histogram displays the lighter pixels, the lack of pixels at this end shows that there are few, if any, whites or very light pixels in this image. What this means is that the image lacks contrast.

What we really want, for most images anyway, is for our images to have tones that go from 0, that is black, to 255, that is white. This will give us a full range of tones. To fix this image, we can either use the Levels or Curves tool to set the highlight and shadow values. This will increase the contrast of the image.

So, let’s start Levels by choosing Levels from the Adjustments panel. We will set the shadow values first. We set the shadow values by moving the black point slider to the right. Notice that the image became darker. This is because the black point slider remaps the tones.  It essentially remaps the tones to a darker tone.

So, how do we know how much to move the black point slider inward? Well, there are several ways that this can be done. The easiest way is what I call the eyeball approach. While not the most accurate method of setting the highlight and shadow values, this approach is quick. Furthermore, this method works quite well for many images, and it will work just fine for us today.

To use the eyeball approach, we just move the black point slider inward to the point where the histogram just starts to slope upward which we have already done. So, our dark tones now start at a tonal value just slightly above 0.

Next, we need to set the highlight values. We set the highlight values by moving the white point slider to the left. Notice that the lighter tones became even lighter. This is because the white point slider also remaps the tones. It essentially remaps the tones to a lighter tone.

So, we move the white point slider to the left to the point where the histogram just starts to slope upward. So, our light tones now start at a tonal value just slightly below 255.

Let’s improve this image a little more by adjusting our midtones. This is done by moving the Gamma Input Slider to the right.

We have one more change that we need to make. Whenever we use Levels or Curves, color changes can occur. Since we do not want this in this image, we need to change the Blend mode of the Levels layer to Luminosity. We can easily do this by choosing the Luminosity Blend Mode from the Blend Mode pop-up menu.

So, we have now set the shadow, highlight, and gamma values. What did this buy us? Well, let’s compare the before and after image. Here is the way the image originally looked. Here is how the image looks after setting the shadow, highlight, and gamma values. Clearly this is an improvement.

At this point, I would like for you to notice something. Look at the Levels dialogue box. Do you see that the RGB channel is selected? This is very important. What this means is that, when we set the shadow and highlight values, we set them in all of the color channels at the same time and to the same setting. In other words, the red, green, and blue channels were all set exactly the same.

This makes perfect sense for this image. How would I know? To answer that question, we need to click off the eye icon of the Levels layer. We now have the image back to its original state. In other words, the way it was before we applied Levels.

Let’s take a look at the histogram again. We are now looking at the RGB histogram. That is, we are looking at all three of the channels together. Now, let’s take a look at the histograms of the individual color channels by clicking on the panel menu and selecting All Channels View.

We now see three color histograms. The important point here is that, for this image, all of the histograms start to ramp up at about the same tone on the left side of the histogram, and they start to ramp up at about the same tone on the right side of the histogram. When we set the shadow values by using the RGB channel, as we did on this image, the shadow values in all three color channels are set the same. Since the shadows start at about the same tone in all three channels, this works pretty well.

Similarly, when we set the highlight values by using the RGB channel, the highlight values in all three color channels are set the same. Since the highlights start at about the same tone in all three channels, this also works pretty well.

Watch what happens when we click the eye icon of the Levels layer back on. We now see what the individual color channel histograms look like with the shadow, highlight, and gamma values set. Notice how each of the three histograms fills the entire histogram area little or no empty areas and little or no clipping of any of the histograms. This is the way we want it for this image.

However, setting the shadows and highlights by using the RGB channel doesn’t always work this well. What am I talking about? Well, watch the next video and you’ll find out.