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While color correcting or adjusting exposure, the histogram can be a great help. 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 amount of tones of particular brightness found in your photograph ranging from black (0% brightness) to white (100% brightness). Ideally, well balanced images will have tonal values across the entire range of the histogram.
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), and to the right are the highlights.
The histogram is able to display Red, Green, Blue channels separately or, by default, shows all of them at once. Click on the Histogram to switch between seeing a composite Histogram or just viewing details about 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.
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 exposed to become absolutely black or white pixels.
Cold Pixels – To enable or disable the display mode of absolutely black pixels, click the triangle on the left and the histogram is clipped on the left side. Absolutely black pixels will be displayed in bright blue in the image. Cold pixels (in blue) indicate areas where black has achieved maximum concentration (a zero value).
Hot Pixels – Clicking the triangle on the upper right will show where your image is completely white, where the histogram is clipped on the right side. Absolutely white pixels are displayed in red.
In both cases this can be problematic (especially for printing) as there is too much ink coverage for cold pixels and no details at all in the hot pixels. These indicators are a sign that you should adjust the exposure of the image. You may want to leave the Histogram panel open as you work, because it is an easy way to learn to read the graphical details of a digital image.
The use of the Develop and RAW Develop filters are an excellent way to take control of the Black and White points as well as the Shadows and Highlights of an image. Pressing the J key will also toggle the clipping indicators On and Off if you want to see the pixels underneath.
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