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Color Histograms

Color Histograms are included in many digital cameras today and offer some distinct advatanges over the standard luminance histogram.

In order to understand why color hisotgrams are an important but often overlooked tool for photographers, we will first discuss what color histograms are. Then we will look at some of the advantages of a color histogram. And finally we will discuss what can be learned from a color histogram and how to use that information to improve our photos.

Understanding color histograpms starts with understanding histograms in general. In photography a histogram is simply a graphical representation of the number of pixels in the image that fall within a certian range, either luminance or color. For example for a normal luminance histogram the graph shows the number of pixels for each luminance or brightness level from black to white. The higher the peak on the graph the more pixels are at that luminance level. With a color histogram the principle is the same but instead of seeing the levels of black graphed you will now see the number of pixels for each of the three main colors.

Learning to read and understand a histogram can be a great way to double check that our photo is properly exposed and to make quick exposure adjustments so we end up with a better exposed photo.

What are color histograms?

A700 HistogramA color histogram is a simply a histogram that shows the color level for each individual RGB color channel. Also known as “Three Color Histograms” these displays are found on some DSLR and high end cameras.

A three color histogram is read similar to a normal luminance histogram. However instead of showing the distribution of pixels from black to white as with the luminance histogram, a three color histogram shows the brightness distribution for each color individually. This is important because you can easily see if one color is overexposed and clipped which means that you might be loosing important detail for that color range.

There are two basic types of three color histograms. The first is the RGB histogram that shows a combination of all three colors and possibly even the luminance histogram all together. The second are individual histograms for each separate color.

What is the advantage of a three color histogram?

The advantage of a color histogram is that you can see if individual colors have been clipped or are over or under exposed.

This is important because it is possible for a luminance histogram to show little if any clipping while a three color histogram would show clipping on one color channel. This would result in a loss of texture or detail for that color similar to what you get with an over-exposed image.

Flower Luminance HistogramThe luminance histogram for this flower picture shows no clipping.

In the photo above you see that th luminance histogram shows no clipping (spikes on either the black or white side of the scale). Yet when you look at a color histogram for the same image you can easily see that the red channel is slightly clipped which means you could be losing some detail in the highlight areas of that color range. There is also a spike on the left hand side of the blue histogram which could mean that the blue channel is slightly clipped in the shadow areas.

Three Color HistogramHere you see the individual histograms for each color as well as a combined histogram that shows all three color channels and the luminance histogram as well. The clipping that is occurring on the red channel is easily visible when using a three color histogram.

Generally having some clipping on just one color channel is not as much of a problem as when all three colors are clipped but it really depends on your image and the look you are trying to capture. That is the case for the flower image above. While the red channel is clipped the image still retains enough luminance detail in the other colors that the clipping is not really an issue in this photo.

What can I learn from a color histogram that is different than a normal luminance histogram?

Color histograms can help you determine if the white balance of your image is correct. If the spike for each color is located at the same place on the histogram for each color channel then the photo is balanced or neutral. This can be a good way of doing a quick check of your white balance but it is important to realize that it does not work on all images. For example an image that contains a lot of one color such as a blue sky will usually have the spike for that color offset even if the white balance is correct.

Color histograms and luminance histograms are important tools for evaluating the exposure of a photo. If we see spiking or clipping on the right hand side of the image we know that the image is likely over-exposed a problem that can quickly be remedied by a quick adjustment to the exposure compensation. On the other hand if the spike is on the left hand side of the image then the shadow areas might be under exposed necessitating an increase in brightness.

Both over exposed and under exposed images mean we are gernally losing valualbe detail and a slight adjustment to our exposure can make for a much better image. Learning to read and use a histogram will help you become a better photographer.

Learn more about histograms at this page on understanding histograms

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