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Transcript for Highlight and Shadow Values by Channel

Video -- Part II

Narration by Ron Bigelow

Photoshop CS4 Used in this Tutorial

In the last video, we set the shadow and highlight values for an image by using the Black Point and White Point sliders in Levels, and we did this using the RGB channel. For that particular image, we got some pretty good results. One might be tempted to just use the RGB channel all the time when setting the shadow and highlight values. However, that would be a mistake. Let’s take a look at why this is the case.

Here we have an image of a mountain in Utah. The image lacks contrast. I strongly suspect that one of the first things that needs to be done is to set the shadow and highlight values. This will help improve the image. Then, the Gamma Input Slider can be used to adjust the midtones.

Let’s launch the histogram. Notice that we have relatively empty spaces on both sides of the histogram (just like we had with the image in Part I of this series). For comparative purposes, let’s quickly set the shadow and highlight values using Levels and the RGB channel. So, we launch Levels, set the Blend Mode to Luminosity, and adjust the Black Point and White Point Sliders. Now, let’s adjust the Gamma Input Slider. That looks pretty good. Why would I bother doing it any other way? Well, let’s see.

Here we have another copy of the original image. Again, we see the Histogram. However, we are looking at the RGB Histogram, and the RGB Histogram doesn’t tell the entire story. We need to look at the individual channel Histograms by clicking on the panel menu and selecting All Channels View.

Now that we are looking at the channel Histograms, we should immediately see a problem. The three channel histograms all look different. The channel histograms all start to ramp up on the left side at different points. Furthermore, the Green channel ramps up on the right side at a different point than the Red and Blue channels. If we make our adjustments using the RGB channel, all of the channels will be adjusted the same. This will not allow us to optimize the contrast in each channel.

On the other hand, we can set the shadow and highlight values separately in each channel. To do this, we start off just as we did before by launching Levels and setting the Blend Mode to Luminosity. As a side note, setting the Blend Mode to Luminosity is very important when setting the shadow and highlight values in each channel. If you forget, you will likely end up with a rather strong color cast. Now, this is where the difference starts. We now go to the Levels dialogue box and select our first channel. Let’s go with the red channel. We adjust the red channel in the exact same way that we previously adjusted the RGB channel. We repeat this process with the green and blue channels. For our final step, let’s go back to the RGB channel and adjust the Gamma Input Slider.

Okay, we are now at the moment of truth. Let’s compare our two versions of the image. First, let’s close the Histograms so that we can see the images better. Here is the image were we used the RGB channel to make the adjustments. Here is the image where we used the individual color channels to make the adjustments. Clearly, the image where we adjusted the individual color channels looks much more dramatic.

So, does this mean that we should always adjust the shadow and highlight values in each of the individual color channels? Absolutely not. For those images where the three color histograms all start to ramp up at about the same point on each side of the histograms using the RGB channel works just fine. Also, for some images, you may not want the extra contrast. On the other hand, it is nice to have this technique available when you need it.

That brings us to the end of this tutorial. However, before I sign off, I would like to take this opportunity to let you know that you can download a copy of the transcript for this video, view several other photography videos, and access over 100 photography articles on my website.