Understanding the Histogram

In this article, I will shed some light on some useful ways to read and understand the histogram attached to all of the images you make. This lesson will describe how to effectively read a histogram as well as provide several examples of how to solve difficult exposure situations.

By no means is the histogram the holy grail of information. If when viewing the graph, you find a well balanced arrangement of pixel information yet your image looks dark, flat or poorly composed, then it most likely is. The histogram is a valuable tool when used properly. You should consider it a means to and end rather than the end itself, meaning you’ll be able to catch flaws and diagnose your images once you understand what you’re looking at.

The most important thing to remember is that the left side of the graph indicates the amount of information in the shadows (dark parts) of the image, while the right side indicates the level of information in the highlights (bright parts) of the image. The middle of the graph represents everything in between.

Below are several examples of how pixels can be arrange across the spectrum of the histogram.

  1. The first graph shows some clipping in the highlights. Notice the spike to the far right? This is a loss of information and comes from blown out highlights. This can occur in a bright sky, from a very bright reflection, a window, or when indoors shooting out through a window.
  2. The second graph shows a loss of information in the shadows. Notice the clipping that is happening to the far left at the very top of the graph? This indicates a loss of information and can occur when shooting in very low light conditions i.e. night shots or when shooting a back-lit subject i.e. a portrait against a sunset.
  3. The third graph shows a collection of information in the middle of the graph. This indicates an overall low contrast image. This often happens under diffused light without the use of a fill flash, or under an overcast, gray sky. To correct this you should re-shoot the image with the inclusion of a hot-shoe mounted fill flash and some reflectors to fill light and add detail.

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This next set of graphs shows other ways the pixel information can be distributed across the histogram spectrum.

  1. The first graph shows a loss of information. Notice the gaps in the graph? This indicates a complete loss of information/pixel data in these areas. This most often occurs during post processing when one opens an image and adjusts the levels and curves to such a degree as to spread out the information. As a side note, the more information you have to start with, larger files sizes (RAW files, HiRes dSLRS 10PM and greater) the greater your post-processing flexibility will be, resulting in less gaping.
  2. The second graph shows a high dynamic range image where the extremes of both the highlights and shadows are being pushed beyond what your camera is capable of capturing. This will occur when shooting a scene like that of a dark forest where sun beams are cascading in through the trees. Or a vast open landscape with a bright sky and dark mountains. To overcome this limitation of your equipment, you can try shooting multiple images of the same scene then combining them into a single finished image. This is referred to as HDR photography and should be used with caution, as this technique is highly over used in today’s digital world.
  3. The third and final graph shows what is arguably a “perfect” histogram. While there really is no such thing as a perfect histogram, this graph shows an even distribution of pixel information across the entire graph. An image with his type of value will result in a wide variety of possibilities once imported into your computer for post-process editing and refinement.

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The important lesson in all of this is that the histogram is a tool for evaluating your images, for setting a baseline and for diagnosing problems. By no means should this tool become the end itself. Keep the histogram handy, become familiar with it, then break the rules as you explore your voice and the endless possibilities that lie ahead yet to be discovered.

~ by John Trefethen

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4 Responses to “Understanding the Histogram”

  1. wren says:

    Great tutorial!

  2. Gene says:

    In regards to the high dynamic range histogram – one solution is a graduated ND filter to knock down the highlights of the sun on the horizon and bring it within the dynamic range of your camera.

  3. trefethen says:

    Great advice Gene, do you hand-hold your graduated ND filter while shooting or do you use a lens mount. Also, what type of filter do you recommend?

  4. Linda says:

    I’ve heard great things about Singh-Ray filters but I don’t have any. They are a bit expensive.
    http://www.singh-ray.com/grndgrads.html