Histogram

Once you have selected an image, you can view the Histogram by clicking View > Show Histogram.

The histogram can be a great help while correcting colors or adjusting exposure. A histogram is a graphical representation of the tonal values of your image. This graph illustrates how the pixels in the image are distributed across brightness levels. In other words, it shows the number of tones of a particular brightness found in your photograph, ranging from black (0% brightness) to white (100% brightness). Well-balanced images will have tonal values across the entire range of the histogram.

1. To read a histogram, start at the left edge, which shows the shadow regions. The middle shows the midtones (where most adjustments to an image are made). Highlights are to the right.

2. The histogram is able to display Red, Green, Blue channels separately or, by default, show them all at once. Click on the Histogram to switch between seeing a composite Histogram and viewing details for the Red, Green, and Blue channels (which can be useful for spotting tint issues and color casts). You can also see a grayscale average for luminance.
3. Additionally, clicking the two small triangles in the upper left and upper right corners will show hot and cold pixels, respectively. These are pixels that have shifted or been exposed to become absolutely black or white.

• Cold Pixels. To enable or disable warnings about absolutely black pixels, click the triangle on the left side of the Histogram. Purely black pixels will now be displayed in bright blue in the image. Cold pixels (in bright blue) indicate areas where black has achieved maximum concentration (a zero value).
• Hot Pixels. Clicking the triangle on the upper right of the Histogram will show where your image is completely white (where the histogram is clipped on the right side). Absolutely white pixels are displayed in red.

*Note: Probematic prints
Cold and hot pixels can both be problematic (especially for printing) as there is too much ink coverage for cold pixels and there are no details at all in hot pixels. The presence of cold or hot pixels is a sign that you should adjust the exposure of the image. You may want to leave the Histogram open as you work because it’s an easy way to learn to read the details of a digital image.

*Note: Check for clipping.
The Light tool is excellent for taking control of an image’s Black and White points as well as the Shadows and Highlights. Pressing the J key will toggle the clipping indicators on and off so you can see the pixels underneath.