Tuesday, November 22, 2011

Understanding Your Camera's Histogram


Using your camera’s histogram screens will guarantee you a much higher percentage of well-exposed images. It is well worth spending time to understand the histogram. It’s not as complicated as it looks.

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.


Light Range

The camera’s sensor can only record a certain range of light values—about 5 to 7 usable EV steps. Unfortunately, many of the higher-contrast subjects we shoot can contain over 12 stops of light values. This is quite a bit more than it is possible to capture in a single exposure. It’s important to understand how your camera records light so that you can better control how the image is captured.

Figure 1 – A basic histogram
 Look at figure 1 closely. The gray rectangular area represents an in-camera histogram. Examine it carefully! Think about it for a minute before reading on.

The histogram is basically a graph that represents the maximum range of light values your camera can capture, in 256 steps (0 = pure black, and 255 = pure white). In the middle of the histogram are the mid-range values that represent middle colors like grays, light browns, and greens. The values from just above zero and just below 255 contain detail.

The actual histogram graph looks like a mountain peak, or a series of peaks, and the more there is of a particular color, the taller the peak. In some cases the graph will be rounder on top, and in other cases it will be flattened.

The left side of the histogram represents the maximum dark values that your camera can record. The right side represents the maximum brightness values your camera can capture. On either end of the histogram the light values contain no detail. They are either completely black or completely white.

The height of the histogram (top of mountain peaks) represents the amount of individual colors. You cannot easily control this value in-camera, other than changing to a Picture Control with more or less saturated color, so it is for your information only.

We are mostly concerned with the left- and right-side values of the histogram, since we do have much greater control over those (dark vs. light).

Simply put, the histogram’s left and right directions are related to the darkness and lightness of the image, while the up and down directions of the histogram (valleys and peaks) have to do with the amount of color information. I repeated this for emphasis!

The left (dark) and right (light) directions are very important for your picture taking. If the image is too dark, the histogram will show that by clipping off the light values on the left; or if it’s too light, by clipping on the right. This will become easier to understand as we look at well-exposed and poorly exposed images. Check out the Histogram Basic Tutorial in figure 2, and then we’ll look at things in more detail.

Figure 2 – Three histograms – one underexposed, one correctly exposed, and one overexposed

When you see the three histograms next to each other, does it make more sense? See how the underexposed histogram is all the way to the left of the histogram window and is clipped mid-peak? Then note how both edges of the well-exposed histogram just touch the edges of the histogram window. Finally, notice how the overexposed image’s histogram is crammed and clipped on the right. I hope this helps somewhat! Now let’s look at some histogram detail.


Histogram Shape

Look at the image in figure 3. It is well exposed with no serious problems. The entire light range of this particular image fits within the histogram window, which means that it’s not too light or too dark and will take very little or no adjustment to view or print.

Figure 3 – Good image with normal histogram shape, no clipping

It contains no more than 4 or 5 stops (EV steps) of light range. To finalize the image, I might increase the brightness in the trees a little, but otherwise it’s a sound image with potential for immediate usage.

Compare figure 3’s histogram to the histogram graph on the left in figure 4. See how the figure 3 histogram does not cram itself against the dark value side, as seen in figurte 4? In other words, the dark values are not clipped off on the left. This means that the camera recorded all the dark values in this image, with no loss of shadow detail.

Then look at the right side of the histogram graph and note that it is not completely against the right side, although quite close. The image contains all the light values available. Everything in between is exposed quite well, with full detail. A histogram does not have to cover the entire window for the exposure to be fine. When there is a very limited range of light, the histogram may be rather narrow.

The image in figure 3 is a relatively bland image with smooth graduations of tone, so it makes a nice smooth mountain-peak histogram graph. This will not occur every time, since most images contain quite a bit more color information. Each prominent color will be represented with its own peak on the histogram graph. The most prominent colors will have higher peaks, while the less prominent will have lower or no peaks.

As we progress into images with more color or light information, we’ll see that the histogram looks quite different.

Figure 4 – Histogram showing underexposure (dark side)

Look at the image in figure 4. This is from an image that exceeds the range of the camera’s digital sensor.
Notice that, overall, this image is dark and looks underexposed. The histogram in figure 4 is crammed to the left, effectively being clipped off. There are no gradual climbs like on a mountain range, from valley to peak and back to valley. Instead, the image shows up on the left side in mid-peak. It is clipped. This is an underexposed image and the histogram reflects that well.

The most important thing to understand when you see a histogram like the one in figure 8.45, with part of the peak clipped off on the left, is that some or all of the image is significantly underexposed.

Now look at a similar image in figure 5. In this image, a larger aperture was used and more light was allowed in. We can now see much more detail. However, once again, the range of light is too great for the sensor, so it is now clipped off on the highlight side (right). The dark-side value is not clipped; instead, the graph extends to the left dark-side edge but stops there.

Figure 5 – Image with highlights (bright side) clipped

The image in figure 5 shows more detail but is not professional looking and will win no awards. The range of light is simply too great to be recorded fully. Many of the details are overly light, and that can be seen by the clipping of the histogram on the right side. The most important thing to remember here is that when you see a histogram graph that is crammed all the way to the right and clipped, some or all of the image is significantly too light. Overall, a great deal of the image in figure 8.46 is recorded as pure white and is permanently gone, or blown out.

It is important that you try to center the histogram without clipping either edge. This is not always possible, as shown in figure 5, because the light range is often too great and the sensor or histogram window can’t contain it. If you center the histogram, your images will be better exposed. If you take a picture and the histogram graph is shifted way left or right, you can then retake the photograph, exposing in the direction of the opposite light value.

If there is too much light to allow centering the histogram, you must decide which part of the image is more important, the light or dark values, and expose for those values.

How Does the Eye React to Light Values? 

The camera, with its imaging sensor and glass lenses, is only a weak imitation of our marvelously designed eye and brain combination. There are very few situations in which our eyes cannot adjust to the available light range. So, as photographers, we are always seeking ways to record even a small portion of what our eye and mind can see.

Since our eyes tend to know that shadows are black, and expect that, it is usually better to expose for the highlights. If you see dark shadows, that seems normal. We’re simply not used to seeing light that’s so bright that all detail is lost. An image exposed for the dark values will look very weird because most highlight detail will be burned out.

Your eyes can see a huge range of light in comparison to your digital sensor. The only time you will ever see light values that are so bright that detail is lost is when you are looking directly at an overwhelmingly bright light, like the sun. So, in a worst-case scenario, expose the image so that the right side of the histogram graph just touches the right side of the histogram window, and the image will look more normal.

Since photography’s beginning, we have always fought with only being able to record a limited range of light. But, with the digital camera and its histogram, we can now see a visual representation of the light values and can immediately approve of the image, reshoot it with emphasis on lighter or darker values, or see that we must use a filter or multiple-exposure high dynamic range imaging (HDR) to capture it at all.

Computer Adjustment of Images 

Looking at the image in figure 6, taken in mid-day overhead sunshine, we see an example of a range of light that is too great to be captured by a digital sensor but is exposed in such a way that we can get a usable photo later.

Figure 6 – Cabin picture with correct exposure but dark shadows, and its histogram

Notice in figure 6 how the dark values are clipped off and dark detail is lost. But look to the right side of the histogram and notice how the light values are not clipped off. The camera recorded all the light values but lost some dark values.

Since our eye sees this as normal, this image looks okay. If we were standing there looking at the cabin ourselves, our eye would be able to see much more detail in the front porch area. But the camera just can’t record that much light range. If we want to get a bit more detail in the shadows than this image seems to contain, we can do it. Normally, a camera does not give us enough control to add light values on the fly, so we use the histogram to get the best possible exposure and then adjust the image later in the computer.

Some cameras can be profiled to capture light ranges more effectively in one direction or the other, but when you push one area, the opposite area must give. So, we need a way to take all this light and compress it into a more usable range.

Figure 7 – Post-processed cabin picture and its histogram (in-computer manipulation)

We are now entering the realm of post-processing, or in-computer image manipulation. Look at the image in figure 7. This is the exact same image as in figure 6, but it has been adjusted in Photoshop to cram more image detail into the histogram by compressing the mid-range values. Notice that the entire histogram seems to be farther right, toward the light side. Also notice that the mid-range peaks are basically gone. We removed a good bit of the mid-range, but since there was already a lot of mid-range there, our image did not suffer greatly.

How this computer post-processing was done is outside the scope of this book, but it is not very hard. Buy a program like Nikon Capture NX 2, Photoshop, Photoshop Elements, Lightroom, or another fine graphics program designed for photographers. Your digital camera and your computer are a powerful imaging combination—a digital darkroom, where you are in control from start to finish, from clicking the shutter to printing the image. But, retreating from philosophy, let’s continue with our histogram exploration. Notice in figure 7 how the histogram edge is just touching the highlight side of the histogram window?

A small amount of clipping is taking place, and you can see the slightly blown out area on the peak of the cabin’s roof. Sometimes a very small amount of clipping does not seriously harm the image.

The photographer must be the judge. The greater apparent detail in this image is the result of compressing the mid-range of the light values a bit in the computer. If you compress or make the mid-range light values smaller, that will tend to pull the dark values toward the light side and the light values toward the dark side. So, you will have more apparent detail in your image. It’s like cutting a section out of the middle of a garden hose. If you pull both of the cut ends together, the other two ends of the hose will move toward the middle, and the hose will be shorter overall. If you compress or remove the mid-range of the histogram, both ends of the graph will move toward the middle. If one end of the graph is beyond the edge of the histogram window (clipped off), it will be less so when the mid-range is compressed.

We are simply trying to make the histogram fit into the frame of its window. If we have to cut out some of the middle to bring both ends into the window, well, there is usually plenty in the middle to cut out, so the image rarely suffers. Remember, this is done outside of the camera in a computer. You can’t really control the in-camera histogram to compress values, but you need to be aware that it can be done in the computer so that you can expose accordingly with your camera’s histogram. Then you will be prepared for later post-processing of the image.

In fact, now that we have compressed the mid-range values, figure 8.48 more closely resembles what our eye normally sees, so it looks more normal to us.

In many cases, your progression from the shooting site to your digital darkroom can benefit if you shoot NEF (RAW) images.

A RAW digital image contains an adjustable range of light. With a RAW image you can use controls in Capture NX2, Photoshop, or even the basic Nikon ViewNX2 software included with the camera to select from the range of light within the big RAW image file. It’s like moving the histogram window to the left or right over all that wide range of RAW image data. You select a final resting place for the histogram window, capture the underlying RAW data, and your image is ready for use.

This is a serious oversimplification of the process, but I hope it is more understandable. In reality, the digital sensor records a wider range of light than you can use in one image. While you might be able to use about 5 stops of light range in a normal image, the digital sensor probably records about 7 stops of light range. Although you can’t get all of that range into the final image, it is there in the RAW file as a selectable range. I prefer to think of it as a built-in bracket, since it works the same way.
This bracketed light range within the image is present to a very limited degree in JPEG, but is the most pronounced in pure RAW images. That is why many choose to shoot in RAW mode instead of JPEG.

Your camera meter should be used to get the initial exposure only. Then you can look at the histogram to see if the image’s light range is contained within the limited range of the sensor. If it is clipped off to the right or the left, you may want to add or subtract light with your Exposure compensation button, or use your Manual mode. Expose for the light range with your histogram. Let your light meter get you close, then fine-tune with the histogram.

There are also other Monitor viewing modes that you can use along with the histogram graph, such as the Highlights (blink) mode for blown-out highlights (see the Playback Menu > Display mode and select Highlights). This mode will cause your image to blink from light to dark in the blown-out highlight areas. This is a rough representation of a highlight-value clipped histogram, and it is quite useful for quick shooting. Using your camera’s light meter, histogram, and the highlight burnout blinky mode together is a very powerful method to control your exposures.

If you master this method, you will have a very fine degree of control over where you place your image’s light ranges. This is sort of like using the famous Ansel Adams’s black and white Zone System, but it is represented visually on the Monitor of your camera.

The manipulation of the histogram levels in-computer is a detailed study in itself. It’s part of having a digital darkroom. Learn to use your computer to tweak your images, and you’ll be able to produce superior results most of the time. Even more importantly, learn to use your histogram to capture a nice image in the first place!

Your histogram is simply a graph that lets you see at a glance how well your image is contained by your camera. Too far left and the image is too dark; too far right and the image is too light. Learn to use the histogram well and your images are bound to improve!

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


8 comments:

  1. This is an excellent guide, I learn a lot!

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  2. I'm glad it was helpful to you, Conrad.

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  3. Wow! Very good info... I guess I'll have to stop ignoring the histograms on my images! Looking forward to better results! Thanks.

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  4. Very helpful and well expressed. Thanks!

    Dino

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  5. Wow I did not know anything about histograms until I read your article. Thank you for the great info and I will use it to improve my photography.

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  6. Thanks Darrell--en"light"ening as usual!

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  7. A graphical representation which shows a visual impression of the distribution of data termed as Histogram . Histogram consists of tabular frequencies which are shown as adjacent rectangles, with an area equal to the frequency of the observations in the interval.

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  8. Well you sure made that a lot easier to understand. Thanks

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