In this video, Blake Rudis from f64 Academy looks at a number of useful aspects of the histogram in both Photoshop (ACR) and Lightroom that you should be aware of.
Just a quick reminder. A color histogram is a graph which is showing the distribution of each primary color’s (RGB) brightness level in your image. Essentially you have three different graphs which can help you understand which color is over or under (a more detailed explanation can be found here).
If you know anything about histograms you probably know that if the graphs are pushed away to the left it means that your image is underexposed. If the graphs are pushed way to the right they are overexposed. If the graph is so much pushed to any of these two directions it means that there might be some pixels with information that you can’t recover.
In general, it is a good working practice to check the histogram just after you shoot to make sure you were not clipping by accident.
Now for the first tip – if you want to turn on the highlight clipping warning in Adobe Camera RAW or Lightroom all you need to do is click on the right or left (or both) triangular icons just above the histogram and they will add red color for overblown areas of your image or blue for underexposed areas. You can also use the shortcuts O and U.
One note about these warnings – the fact that you have a warning in a certain part of your image doesn’t mean that you necessarily have to do something about it. You might want that over or underexposed area in your shot, so use this feature when you need it but always remember what your goal is with the edit.
Editing with the Histogram
A Histogram is typically a tool for viewing exposure levels, however as Rudis explains in the video, you can also use it to edit the image. In fact, if you hover with the mouse over the histogram in ACR/lightroom you will see that you get the following options (left to right):
By clicking and moving the cursor left and right in each zone you can change the levels of that option and basically edit from the histogram itself. We don’t find this particularly convenient or accurate but it’s always good to know.
Converting to Lab
If for some reason you want to know the specific RGB value of a specific pixel you can do this by looking at the values displayed on the histogram while your mouse is hovering over this pixel.
If you prefer to view these values in lab instead of RGB you can do that by right-clicking on the histogram and choosing the show lab option.