Join Tim Grey for an in-depth discussion in this video Reading a histogram, part of Photoshop: Mastering Color Correction (2012).
You may be accustomed to evaluating the histogram on the back of your digital camera when you're checking your exposures for example. But you might not be in the habit of checking the histogram in Photoshop. And actually, evaluating the histogram can be helpful in terms of evaluating some of the color information about an image. Let's take a look at the histogram display within Photoshop. To get started, I'll choose Window, followed by Histogram from the menu. To bring up the Histogram panel, if it's not already visible. You'll notice right away that we have some color information presented.
We also have a warning indicator. There's an exclamation point and a triangle. And what that means is that the current histogram that's being displayed is based on cached data. It's not based on the actual pixel values within the photo. Quite honestly this is rarely an issue you need to concern yourself with. It's just helpful top know exactly why that warning symbol is there. If you want to make it go away and refresh the histogram in the process you can simply click on that warning icon. In most cases you'll notice a very slight shift in the histogram display itself.
Now that I've taken care of that, let's consider some of the options for changing how the histogram is displayed. I'll click on the panel popup menu, for the Histogram panel. And from the popup, I'm going to choose Expanded View as a starting point, that will give me a larger display, and it gives me some additional information about the photo, here. Unless you're a mathematician, much of this info is probably not meaningful. Nor helpful, but there is another display option that can be very helpful. And that's to view all of the individual channels individually.
I'll go ahead and click that panel pop up menu one more time. And I'll switch to All Channels view. And now you can see that I have not just a composite histogram but also individual red green and blue histograms. Now if you take a look at these individual histograms and then compare them to the histogram we're seeing up above. You'll notice some significant similarities and that's because the colors histogram that's displayed by default up at the top of the histogram panel is exactly the same things. Its just those individual three channels stacked one on top of the other. And in fact each of those channels is also displayed in a color that represents that channel.
So the red channel is presented as red. The green channel is presented as green and the blue channel of course is presented as blue. You'll notice, of course, that we're not seeing only red, green and blue. And that's because some of these channels are overlapping. In fact, all of them overlap in certain places. And those areas are indicated in gray. Other colors, for example yellow indicate where just a couple of the channels are overlapping. So you can evaluate the overall image by looking at the colors histogram display. But in some cases you might find it easier to evaluate the individual histograms.
Let's take a look at a couple of examples of some of the information that we can gather from the histogram display. To start with, we don't have a histogram that goes all the way over to the right side and that indicates that there are no pixels in the image that have an actual white value. But notice that the brightest value for each of the historgrams shown here, red green and blue don't line up. The red histogram goes way over to the right, almost to white. Whereas the blue histogram stops at right around a middle grey value in terms of luminance.
That's telling us that the brightest areas of the image are not neutral. They are not a shade of grey, not a value of white for example. But are shifted toward red. By contrast, the shadow areas are relatively neutral. You can see that the green values are shifted a little bit more toward the right. More toward the brighter value as compared to red and blue. But the point is that the shadows are a little more neutral not exactly neutral but closer to neutral compared to the highlights. We can also see that the red channel really constitutes the majority of the image which tells us that the image contains a lot of red tones and cyan tones since red and cyan are opposites. And therefore represented mostly by the red channel itself. And looking at the image you can certainly see there is a fair amount of red pixels within the image or pixels that at least shift a little toward red. And we also have a fair amount of cyan tones in the shadows and that doesn't necessarily mean that all we see are red and cyan values it just means that the colors are shifted in those directions a little bit. So for example the greens in the shadows have a little bit of a bluish tend to them.
They're a little bit cyan as oppose to being a pure green. And of course we can determine that information by looking at the colors histogram display as well. With time you might find that the color display is easier to evaluate because you have a little bit more information displayed in a smaller space compared to looking at the individual red green and blue channels. You can also choose a variety of different options for channel. RGB is one that I do not recommend. This is just showing the overall composite RGB data. It's not the same as the luminous chart for example.
It's just red green and blue values mixed together so I don't reccommend that. You can view the individual red, green, and blue channels at the top here. Obviously if you had the RGB channels displayed below then you wouldn't need to switch between those individual channels. But if you're using the smaller histogram display without thoe individual color channels you could switch through the channels this way. Luminosity can sometimes be helpful because it shows us the overall distribution of tonal values within the photo, but for my money, I think the colors display is the best option by far.
It gives us the most information that we're truly interested, in terms of evaluating our image. So as you can see, it can be very helpful to evaluate the hystogram, not just when you capture the image to check exposure, but also as you're evaluating colors while you're optimizing your photo.
- Configuration considerations
- Evaluating color
- Basic color for raw images
- Essentials of color balance
- Vibrance vs. saturation
- Adjusting temperature in Lab mode
- Strong color cast removal
- Focused color corrections
- Color matching