Using the Histogram Panel in Adobe Photoshop Lightroom 4
When you are in the Develop module, the Histogram panel is displayed in the top-right corner (there is also a Histogram panel in the Library module, but the histogram in the Develop module has more direct relevance when making Develop adjustments). Basically, the Histogram panel provides you with information about the distribution of the levels in an image and also offers you the means to turn the clipping previews for the shadows and highlights on or off—these can indicate where there might be any shadow or highlight clipping in the image. You can either roll over or click on the buttons circled in Figure 4.31 or press to toggle displaying the clipping preview shown below. Blue indicates where there is shadow clipping and red indicates any highlight clipping. The clipping warning triangles themselves also indicate which colors in the red, green, or blue channels (or combination of channels) are initially being clipped most—the triangle colors will eventually change to white as all three channels become clipped. If you want to hide the Histogram panel you can use the (Mac) or (PC) shortcut to toggle collapsing and expanding this panel.
Figure 4.31 This shows the Histogram panel with the clipping warning triangles highlighted. With this particular image the clipping preview shows red and blue channel clipping in the shadows and red color channel clipping and highlight regions.
If you are editing an imported JPEG, PSD, or TIFF image, the Lightroom histogram represents the tone range based on the file’s native color space. If however you are editing a raw capture, there are no gamut constraints until you export the image as a JPEG, TIFF, or PSD file, at which point the gamut space limit is determined by the choice of RGB output space. sRGB has a small gamut and many of the colors will be clipped when you export. Adobe RGB is a popular, commonly used color space, and ProPhoto RGB has the widest gamut of all. Incidentally, Lightroom uses a wide gamut RGB space similar to ProPhoto RGB to do all the image calculations, and the histogram and RGB percentage readouts are based on this native Lightroom RGB space. To find out more about the Lightroom RGB space, please refer to the book’s website.
The Histogram panel and image adjustments
As you adjust an image you can observe how this will affect the image levels in the Histogram panel. In Figure 4.32 you’ll notice how as the Exposure was increased the levels expanded to the right. As the Exposure amount was increased more the highlights didn’t clip any further and the midtones became brightened. But if you push the Exposure adjustment to extremes the highlights will eventually be forced to clip.
Figure 4.32 In the Histogram panel view on the left, the highlights needed to be expanded to fill the width of the histogram. It was also evident in the image itself that the Exposure needed to be increased. In the example on the right, I adjusted the Exposure slider to make the image brighter.
You may also find it useful to reference the Histogram panel when adjusting images like the one shown over the page where an image initially appears to be overexposed. In examples such as this, as you decrease the Exposure slider setting more information should appear in the highlights and this will be reflected in the histogram display (see Figure 4.33). But note that the ability to recover highlight detail in this way only really applies when processing raw images.
Figure 4.33 In this example the Histogram panel view on the left shows the histogram of an overexposed image with shadow and highlight clipping. The example on the right shows how the histogram looked after applying negative Exposure and Highlights adjustments to recover detail in the highlights that would otherwise have remained clipped.
What is interesting to note though is that the histogram is more than just an information display. You can also use it to actively adjust the following Basic panel tone slider controls: Exposure, Highlights, Shadows, Whites, and Blacks. As you roll the mouse over the histogram, you’ll see each of these sections highlighted in the histogram (Figure 4.35). And if you click and drag left or right with the mouse inside the Histogram panel you can use this as an alternative way to adjust the Basic panel sliders.