If you just started in photography or began using Photoshop or Lightroom, the chances are you have already seen the graph that is called the Histogram. You might not have given it too much thought up to this point, but if you are readying this tutorial, it is safe to assume you would like to know what exactly it shows or does. This tutorial will be your comprehensive guide on how to use the histogram in Photoshop. It will attempt to tackle some of the misconceptions surrounding the histogram, teach you how to read it and how to utilize it in your photo retouching workflow.
The histogram is usually visible in the top right corner of your Photoshop workspace, if it is not, enable it by going to the ‘Window’ section of the Photoshop toolbar and find the histogram in the drop down menu that appears.
If you do not have an image open, it will be empty. Once you open an image, it will show a graph representing the pixel distribution over the tonal range of the image starting at pure black on the left side and going to pure white on the right side. The higher the density of pixels are located at a specific tonal range the higher the peak of the graph for that region.
How the histogram looks, will depend largely on your image. If you have an image that has a wide range of tones taken in a relatively diffused lighting conditions and exposed correctly, the histogram should be distributed fairly evenly. It should display pixels on the far left as well the far right, such distribution is an indication of a strong contrast ratio; however, it is often misconceived that this is the rule that all your photos should follow. Unfortunately, there is no such thing as the correct or the right histogram that photographers should aim for. It is merely a tool, but a useful one that can help you better understand the images you are working with, how they can be changed or cannot be changed.
The example image underneath, is a concert photo that is dark overall, it is not underexposed, but rather reflects the way the scene looked at the point the image was taken. It is not unexpected that most of the histogram for the image falls on the left side of the curve. You will also notice that on the right side of the histogram where the white tones are located, the curve is actually clipped, meaning that there are parts of the image area that are pure white and the detail has been lost in that area. It can be interpreted as an indication that there is an issue with the images; however, in the case of this image it does not affect the subject of the image, which is the crowd and the performers; the detail that is actually lost only affects the bright areas of the stage lights – there is no need to worry about exposing the stage lights correctly, and in fact it will prove to be impossible if you want your entire scene to be exposed properly. To summarize, this example shows that the histogram can only be relevant to one particular image – applying a general rule to distribute the curve evenly for the example photo would render it unusable.
There is one option of the histogram in Photoshop that is often left untouched, but can be a great tool to further help you understand your image – that is the option to see the histogram for separate channels of Red, Green and Blue. To extend the histogram to show those channels, click the icon to open the options menu and click ‘All Channels View’ option. It will open up three separate histograms displaying the tonal range for those specific color channels.
There are a number of reasons why this tool can be particularly useful. First, in reference to the example image of a bedroom, the single histogram may show the highlights being clipped, which might be concerning, but if you open the channel histograms you will notice that it is only clipped in the red channel, likely caused by the reddish colored lamps in the image. Having some clipping occur on the lamps is perhaps not as important as exposing the room correctly, therefore, with the help of color histograms we can make the judgment to leave the highlights clipping.
The color channel histogram can also help you check, which images you will not be able to correct due to highlight clipping. For instance, the concert shot underneath taken in predominantly red lighting appears to look more or less correctable by applying slight color hue and saturation adjustments. However, if we look at the red color channel histogram, we can see that there is significant highlight clipping.
We can quickly check what the highlight areas are by using the Levels adjustment tool. In the example image it reveals that the highlights, which are the areas where the detail is being lost, are the musician’s face and the guitars – both focal points in the image. This means that it will be practically impossible to remove the red cast from the image since there is not enough color information remaining in those areas – the best bet in this type of scenario if we want to still use the image is to simply turn it into black and white.
A good way to think of the histogram in Photoshop is to consider it an image map that tells you what is happening in specific areas of that image; however, it is still most important to remember that the photographer or the editor is still the only reliable judge to tell what the exposure of the image should be and the histogram is merely a tool to help make that judgement.
Did you find the tutorial useful? Learn further by reading our tutorial on how to correct overexposed photos in Lightroom.