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Understanding Histograms

Understanding Histograms

  • What are histograms?
  • Why are histograms important?
  • How to use histograms?

This page will help you answer those questions and begin understanding histograms and why they are important.

What are histograms?

A histogram is a visual picture that shows how many pixels of a particular brightness level there are in the photo. It is like a graph that shows you how much light each pixel of your image sensor is reading.

There are two basic types of histograms. The first type is the standard luminance histogram which is the focus of this page and the second type are RGB or Three Color Histograms.

Normal HistogramThis is an example of a well-balanced histogram. Notice the small spikes at both sides indicating the picture has small areas of pure black and pure white but the majority of the pixels are in the center to middle area indicating excellent midrange detail. It also has broader "peak" areas indicating good contrast.

The purpose of the histogram is to let you see instantly if your photo is correctly exposed. Once you learn how to read a histogram it gives you instant feedback as whether your photo is correctly exposed that can be more accurate than simply looking at the screen.

Why are histograms important?

Understanding histograms begins with understanding why they are important and what purpose they serve. Histograms are important because even though today’s cameras have excellent metering systems there are lighting conditions that can “fool” the cameras internal light meter resulting in an incorrectly exposed image. For example if your subject has a large bright area the camera meter will think it needs to reduce the amount of light which can result in an under-exposed image. On the other hand if you have a large dark area in the photo it can fool the camera and result in an over-exposed photo.

Now you might ask why can’t I just look at the LCD screen and tell if the photo is exposed correctly? The answer is you might be able to some of the time but why chance it when you have a tool that takes the guesswork out of making that judgment right at your fingertips.

One problem with relying on your LCD display alone is that it can sometimes fool you. For example if you have your LCD display set to the maximum brightness a photo might look over-exposed when it really is not. Also there are times when viewing the LCD in sunlight that the screen looks washed out or is hard to see due to the bright sun. Finally you have the fact that even the largest LCD’s on cameras are really too small to accurately judge a photo’s exposure and color.

So with all these factors working against you why not check your trusty histogram and know for sure how well your photo is exposed. The fact is there are times when you really can’t tell how well the picture is exposed by looking at the LCD display….and it is in those situations that your histogram helps you the most.

How to use histograms?

Knowing how to use histograms is an important part of understanding histograms and begins with understanding how a histogram is set up.

Histograms are set up so the darker pixels show up on the left hand side and the lighter pixels are found on the right hand side. If most of your pixels are found on the left hand side your image is likely under-exposed and will be too dark and lack detail in the shadow areas. If the majority of the pixels are found on the right hand side then the photo is likely over-exposed and will be too light with little or no detail in bright parts of the image.

Understanding Histograms Through Examples

Histogram Normal ExposureThis is a normally exposed image. Notice how the histogram covers from true black to true white with little clipping. This image is using the full dynamic range of the camera.

 Over Exposed HistorgramThis is an over-exposed image. Notice how the histogram is shifted to the right and you have a large spike on the right hand side. This indicates that the highlights are clipped and contain no detail. Also notice how the histogram is "flatter" which indicates less contrast in the image.

Under Exposed HistogramThis is an under-exposed image. Notice how the histogram is shifted to the left and you have no white pixels at all in the image.

Pixels on the far left hand side of the histogram represent pure black or a light intensity level of 0 while pixels on the far right hand side represent pure white or a light intensity of 255. Pixels between these two areas make up the areas of the image that contain detail or the tonal range of the image.

Histograms typically look like a mountain peak or a range of peaks. Those areas of the photos that have the greatest numbers of pixels for that luminance or color level will have the tallest peak.

Understanding Histograms can also help us see the contrast of the image. An image with a low contrast will generally have a “steeper” peak on the histogram while an image with more contrast will have more of a rounded peak to the histogram.

Contrast histogramHere are two examples showing the difference contrast makes in a histogram. The picture on the left is a low contrast picture that looks "flat" as seen by the narrow and steep peak in the histogram. The picture on the right had the contrast adjusted and notice how the histogram spreads out resulting in a broader peak and a better photo.
It is important to realize that there is no perfect histogram. The right histogram for one scene or one picture might not be right for the next scene or picture. There are times when the best histogram for an image might be heavily weighted towards either the light or dark side of the histogram and yet the exposure is correct for that image and what you are trying to capture. An example of this would be a photo of the moon at night where almost all your pixels will be on the left hand side of the histogram but you image is properly exposed. 

Using a Histogram to Help Adjust Your Camera Settings

Histograms allow us to quickly see how well our image is exposed. Once we know how to "read" a histogram and understand what it shows, the next step is to understand how to adjust our camera settings in order to avoid under-exposing or over-exposing the picture.

One quick way of adjusting the exposure slightly up or down to improve your histogram and image is by using the "exposure compensation" feature of the camera. This camera setting allows you to quickly adjust your exposure either up or down slightly to either lighten or darken the image.

Histograms Reviewed

A histogram is a graph or visualization of how much light your image sensor is capturing at each pixel location. There is no “prefect histogram” and a histogram alone cannot tell you if your picture is overexposed or underexposed, it only tells you how your image is exposed. There are times where a histogram might look like the image is under or over exposed when in reality it is not, because of the subject matter.

For example some images are simply darker because most of the details or tones are found to the left side of the histogram in the shadow areas. These images are called “Low Key” and the histogram will show most of the pixels on the left hand side. The opposite of this type of image is the “High Key” image where most of the detail or tones are in the highlights of the image. This type of picture will show a majority of the pixels on the right hand side of the histogram.

For most cases the histogram is a very accurate way of making sure your image is properly exposed and you are not losing important details in either the shadows or highlight areas of the picture.

Balanced HistogramAnother example of a well exposed image and a balanced histogram. We know by looking at the histogram that we have all levels of light represented from true black to true white and that the picture has good contrast by the wide peak of the histogram.

I hope you have found this page on understanding histograms helpful and I wish you the best as you continue down the path of becoming a better photographer.

Click here to learn about color histograms and their advantages

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