A Deeper Look at the Histogram

This is a short excerpt from my upcoming book Moving Beyond Point-and-Shoot Photography due in March 2012. The book is designed to help enthusiastic new DSLR and ILC/EVIL camera users learn how to shoot well with their new cameras. It assumes no previous knowledge of photographic terms, principles, or technology.

Back in the “good old” film days we didn’t have a histogram, so we had to depend on our experience and light meter to get a good exposure. Since we could not see the exposure until after we had left the scene and developed the film, we measured our success by the number of correctly exposed images we were able to create. With the exposure meter/histogram combination found in your camera the good exposure success rate you can experience is much higher than ever before.

Is the Histogram Really That Important?

The histogram can be as important, or even more so, than the exposure meter. The exposure meter sets the camera up for the exposure, and the histogram visually verifies that the exposure is a good one. Together they will give you the most accurate exposures you have ever made, if you use them both.

If your exposure meter stopped working, you could still get excellent exposures using only the histogram. In fact, I gauge my efforts more by how the histogram looks than anything else. The exposure meter and histogram work together to make sure you get excellent results from your photographic efforts.

Figure 1.1 – Two histogram types (Luminance and RGB)

Figure 1.1 shows two histogram types from my Nikon D7000. The first screen in figure 1.1 shows a series of histograms to the right of the small picture of my grandson and me. On top is a white-colored luminance (brightness) histogram, followed by individual red, green, and blue channel histograms (RGB = red, green, blue). On the second screen, the luminance histogram appears to the right of the small picture of my cars in the snow.

I have no way of knowing whether your camera offers only a single luminance histogram, like the one in figure 1.1’s second image, or whether it gives you a RGB histograms too, as in figure 1.1’s first image. What is the difference between the luminance and RGB histograms? Let’s examine both histogram types and see.

RGB Histograms

The RGB histograms show all three color channels that a camera uses—on an individual basis. Remember, the camera combines the red, green, and blue colors from its color channels to make the final color in the picture. The red, green, and blue colors are blended together to provide color in up to trillions of shades, well representing the colors your eyes see in your subjects. Therefore, the RGB histograms are simply representations of how well your camera exposed each basic color that it later combined into the final image.

Luminance Histogram

How does the luminance histogram differ from the RGB histograms. The luminance histogram is a representation of the perceived brightness (luminosity) from the combination of the red, green, and blue channels shown in the individual RGB histograms. In other words, the luminance histogram tries to accurately reflect the light you actually see by weighting its color values in a particular way. Since the human eye sees green most easily, the luminance histogram is heavily weighted toward green. Notice in figure 1.1’s first image how the luminance histogram on top looks very similar to the green channel histogram below it. Red and blue are represented in the luminance histogram too, only in lesser quantities (59 percent green, 30 percent red, and 11 percent blue = luminance). The luminance histogram measures the perceived brightness in 256 levels (0–255).

In my opinion, the luminance histogram is a more accurate way of looking at the color levels in real images. Since it more accurately reflects the way our eyes actually see color brightness, it may be the best histogram for you to use. Now, let’s discuss the use of a histogram in detail.

Understanding the Histogram 

Finding and using your camera’s histogram(s) will guarantee you a much higher percentage of well-exposed images. It is well worth spending time to understand the histogram. I’ll try to cover this feature with enough detail to give you a working knowledge of how to use the histogram to make better pictures. If you are deeply interested in the histogram, there is a lot of research material available on the Internet. Although this overview is brief, it will present enough knowledge to improve your technique immediately.

I am going to concentrate on the luminance histogram. It is the best histogram for most photographers to use since it accurately reflects the way we see light. I am not going to keep on repeating luminance histogram over and over. From this point forward, when you see the word histogram, realize that I am talking about the luminance histogram.

What is The Basis for a Histogram?

When you take a picture, whether in JPEG, TIFF or RAW mode, the camera presents the luminance histogram based on its approximation of a JPEG image. In other words, the histogram is what the camera or computer would show for an 8-bit JPEG image (256 color levels per RGB channel).

When you take a JPEG (.jpg) picture the camera crams all the light values of the RGB channels into 256 levels. The same thing happens when you take a picture in 8-bit TIFF (.tif) mode. All the light values are reduced to 256 levels. When you shoot a RAW image, there are significantly more than 256 color values available.  However, the camera still shows you a JPEG histogram when you are viewing a RAW (.nef) image on the camera’s monitor. In reality, most 12- or 14-bit RAW images can hold from 4096 to 16384 color levels per channel. However, all that color is represented by a 256-color-level-per-channel histogram.

In a way, this is a safety factor for RAW shooters. A RAW image has additional capacity to record light values within the brightest parts of the image (highlight headroom). The camera does not show you the histogram based on the total capacity of the RAW image. It uses a JPEG image as the basis for the histogram. For 8-bit JPEG and TIFF shooters, the histogram gives you exactly what you see and nothing more.

Therefore, if you shoot mostly in JPEG or TIFF, be careful that the histogram is exactly right or you may have badly exposed images. For RAW shooters, the histogram under-represents the actual highlight headroom you have available in the image; however, if you shoot for an accurate histogram anyway, you will have less noisy images, even in RAW, because the limited exposure range of the JPEG-based histogram fits well within the headroom of a RAW image. A RAW shooter just has more room to correct errors in exposure since greater range is available in the image than the histogram shows. As a RAW shooter, I always check the histogram for my best images.

The main point I want to make in this article is use your camera's histogram. Your pictures will be better for it!

Keep on capturing time...
Darrell Young
See my Nikon books here:
http://www.photographywriter.com/NikonBooks.asp