Editor’s Note: The following is an excerpt from a new book called the Photofocus Guide: Develop Great Images in Lightroom. This book is almost done and we’ll be giving away free copies soon to all our readers thanks to Mosaic. Be sure to check out the Lightroom Learning Center.
The Histogram panel contains a number of tools to help you evaluate your photos exposure and even begin making adjustments. The most visible part of the panel is the histogram itself, which is simply a graphical representation of all of the tones contained in your photo, from the darkest tones on the left to the brightest tones on the right. While there is no such thing as a good or bad histogram, the histogram can be incredibly instructive when examined alongside your photo. The histogram below shows that the associated photo contains brightness values that span across the entire tonal range, from pure black on the left to pure white on the right.
Reading the Graph
The colors you see in the graph represent the red, green, and blue color channels. Areas of gray occur where image data from all three channels overlap, while areas of yellow represent overlap of the red and green channels, areas of magenta occur when blue and red overlap, and areas of cyan represent overlap of the green and blue channels.
Heres a tip, the histogram always reflects the area of the photo inside the crop rectangle, so sometimes it is worth starting your adjustments by cropping out any unwanted areas of highlight or shadow clipping on the edges of the photo. This way the histogram will reflect just the data you are keeping, which will make your job easier when you are performing basic tonal adjustments.
When your cursor is over the photograph you will see the percentages of red, green, and blue contained in the pixels under the cursor displayed below the histogram. When the cursor is not over the photograph you will see exposure information from the photos EXIF metadata displayed below the histogram.
Checking for Clipping
Clipping means that there are areas in your photo that contain no image data. This can happen in the shadow region or the highlights or even both at the same time. Clipping on the histogram is represented by spikes along the left or right edges. This histograms is from an over exposed photo that has lost all detail in the highlights.
This histogram is from an under exposed photo that has lost all detail in the shadows.
The triangles in the upper left (shadow) and right (highlights) corners of the histogram are clipping indicators.
- When all three channels are being clipped they turn white, but if you see a color in the indicator than only one or two channels are being clipped.
- You can get a real time view of where this clipping is occurring in your photo by placing your cursor over on an indicator.
- Click that clipping indicator to keep the clipping preview turned on. Areas in your photo where the shadows are being clipped will turn blue, while areas in your photo where highlights are being clipped will turn red.
- You can also toggle the clipping indicator preview on and off by pressing the J key. Leaving these indicators enabled while you make tonal adjustments can be very helpful.
We’ll continue this series on histograms in a followup post next week.
Rob writes the “Under the Loupe” column for Photoshop User Magazine, and is the author of many photography related books.