Digital photography offers certain benefits for thinking photographers that are only realized with a basic understanding of a metering feature called the "histogram."
Using the histogram in your camera's image viewing LCD will guarantee you a much higher percentage of well exposed images. It is well worth working to understand the histogram, and is not overly complicated. I will 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 much research material available on the Internet. This article is only covering a small amount of the information that is actually available, but will present enough knowledge to improve your technique immediately.
Nikon D2X rear view
A digital camera sensor can only record a certain range of light values. Remember how with film the light range was limited to only a few f/stops of range? With transparency film the limitation was about 3 or 4 "stops" of light, while with negative film it could go on out to 5 or 6 stops. Today's digital sensors seem to be able to record about 5 or so usable stops of light. Most will agree that the digital camera can record a little more than transparency film, and a little less than negative film. This will change, since some digital cameras are now offering extended range sensors.
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 with film or a digital sensor today. It is important to understand how your digital camera records light, so that you can better control how the image is captured.
The histogram basically is 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. The more of a particular color, the taller the peak. In some cases the graph will be rounder on top, or flattened. The left side of the histogram represents the maximum dark values that your camera can record. The right side represents the maximum white 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 top of the histogram (top of mountain peak) represents the number of different colors, a value you cannot control, 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 control over those. (Dark vs. Light)
So, basically, the histogram's left to 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 color information. I repeated this for emphasis! The left (dark) to right (light) directions are VERY important for your image making. If the image is too dark, the histogram will show that by clipping off the light values on the left, or, if too light, by clipping on the right. This will become easier to understand as we look at well exposed and poorly exposed images.
Look at the image below (Figure 2). 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 is not too light or too dark, and will take very little or no adjustment to view or print. It contains no more than four or five stops of light range.
Look at the left side of the histogram graph above (Figure 2), and see that it does not cram itself against the dark value side. 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 darker 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, such as the blues and grays, are all 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 2 is a relatively bland image with smooth graduations of tone, so it makes a nice smooth mountain peak looking histogram graph. This will not happen all that often, 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 will see that the histogram looks quite different. Look at the image in Figure 3 below, which is one that far exceeds the range of the camera's digital sensor.
Notice that, overall, this image is dark and underexposed looking. The clouds are pretty well exposed, but the image is not very usable unless the clouds are the primary subject. See how the histogram above (Figure 3) is crammed to the left, effectively being clipped off there? 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." (Remember that word) If this is confusing, refer to the histogram graph (Figure 1) at the very top of this article, and notice that it has unclipped peaks and valleys like a mountain range.
If you don't fully grasp this yet, do not worry. The most important thing to know is that when you see a histogram like in Figure 3 above, with part of the peak and valley clipped off on the left, THE IMAGE IS TOO DARK. This problem could be corrected on this image by using a neutral density filter on the sky, which would have compressed the light range enough that the image could be more fully recorded. This image above is even clipped a little on the highlight side (right). You can see why when you look at the rays of light shining between the clouds. The light is too bright, so it exceeds the light range of the sensor and is clipped.
IMAGE AND HISTOGRAM SHAPE ... continues
Now look at a similar image below (Figure 4). In this image a larger aperture was used and more light was allowed in. We can now see the ground, but, 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 graph value is not clipped; instead the graph extends right to the left dark-side edge but stops there.
This image in Figure 4 above shows more detail in the ground area, but it is not professional looking, and will win no awards. The range of light is simply too great to be recorded fully. The clouds and light behind them are overly light, and that can be seen by the histogram's clipping on the right side. The most important thing to remember with this image's histogram is that when you see a histogram graph that is crammed all the way to the right and clipped, THE IMAGE IS TOO LIGHT. Overall, a great deal of the image in Figure 4 is recorded as pure white and is gone permanently. (It is "blown out")
Also notice in the Figure 4 above that there are few mid-range values, as represented by a big valley in the mid-range area. It has two peaks, the left representing darker values and the right representing lighter values. There is no strong mid-range peak. Of course, other images will have multiple peaks and be just fine. The important thing is that you prevent the image's light values from being clipped on the left or right. This is not always possible, but do your best to try. In other words, if you try to center the histogram, your images will be better exposed. If you take a picture, and the histogram graph is shifted way left or right, well, you can then retake it, 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. Does that make sense? You must expose for the highlights, or you will lose detail in the light areas. Which is more important, the dark areas, or the light areas?
HOW DOES THE EYE REACT TO LIGHT VALUES?
The camera, with its lenses, film, or sensor is only a weak imitation of our marvelously designed eye and brain combination. There are very few situations where our eye cannot adjust to the available light range, and we can see well. So, as photographers, we are always seeking ways to record even a small portion of what our eye and mind can see.
Since our eye tends to know that shadows are black, and expects 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 will look very weird because all highlight detail is gone. Your eye 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 worse 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. (See the next image for a view of this)
This is no different than shooting with film, since 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 to capture it at all.
Looking at the image below (Figure 5), 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.
Notice in the image above (Figure 5) how the dark values are indeed 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. So 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, the 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.
We are now entering the realm of "post-processing," or of out-of-camera (in-computer) image manipulation. Look at the image below. This is the exact same image as above, 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.
A side note: How this computer post-processing was done is outside the scope of this article, but it is not very hard. Buy a program like Photoshop®, Photoshop Elements®, Micrografx Picture Publisher®, 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 histogram exploration.
In the image above (Figure 6), see 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 midrange 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 of the ends of the graph is beyond the edge of the histogram window, or is 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 being 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 "postprocessing" of the image in your computer.
In fact, now that we have compressed the mid-range values the above image more closely resembles what our eye normally sees, so it looks more normal to us.
This is a strong benefit of digital vs. film photography. Most do not realize it, but a RAW digital image contains an adjustable range of light. In a sense, the camera records a bracketed light range inside of a single image. Each RAW image contains about a three stop bracket. It would be like taking three slides; one exposed a stop under, one in the middle, and one a stop over, then combining them into one image. With a RAW image you can use a slider in Photoshop®, or your camera's included RAW image computer conversion software, to select from the bracketed range of light within the big RAW image file. It is 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 then your image is ready for use. This is a serious oversimplification of the process, but, I hope, makes it 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 is recording probably about seven stops of light range. You just 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, and a bit more so in TIFF images, but is the most pronounced in pure RAW images. That is why many choose to shoot in RAW mode, instead of JPEG or TIFF. If you are unsure about these modes, please read this linked article for a better understanding.
Most digital cameras have a rocker switch of some sort that is used to scroll through the images you have already taken. Most of these rocker switches will be pressed up or down to scroll through the images, and then left or right to select various informational modes, such as the histogram mode. So, when you take a picture of an important subject, use the little thumb rocker on the back of most digital cameras to select the histogram view of your image. (Check your manual, if you can't figure it out)
Your camera meter will be used to get the initial exposure only. Then you will look at the histogram, and 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 +/- EV 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.
In Figure 7 above is an actual histogram as seen on the back of a digital SLR. Note that the histogram is not clipped on either the dark or light sides. The yellow histogram graph is well centered, so the image is well exposed.
Notice also that there is not much mid-range in this image (Figure 7). It is rather high-key, since it is a direct flash shot with a light background. See how the histogram shows this by peaking the mountain range very sharply in the lighter side of the histogram window. The tallest peak is probably a representation of the white background, while the smaller peak to the right is most likely a representation of her skin tones. The young lady's hair is mid-range to dark, so you see a smaller peak to the left side. See how nice it is to be able to use the histogram to control your exposures?
Most of us will try to expose for the bright light, since our eye craves to see detail in the light areas, but could care less about detail in the dark areas. Keep your histogram's bright light values side from being clipped off, but get it really close, and your image will probably look a lot better. If the light is too contrasty, then you must resort to filters and reflectors, or overcast skies, like in the good old film days.
There are other LCD viewing modes that you can use along with the histogram graph, such as the blinky blinky mode for blown out highlights, which most digital cameras have today. What this does is to blink from light to dark in the blown out highlight areas. This is a rough representation of a highlight-value clipped histogram, and is quite useful for quick shooting. Using your camera's light meter, histogram, and the highlight burnout blink 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 Black & White Ansel Adams "Zone System," but it is represented visually on the LCD of your digital camera.
You can even use your histogram to imitate your favorite film type. For instance, let's consider transparency film. It is limited to only a few stops of light and is very sensitive to overexposure. A digital sensor is somewhat similar. So, if you use the histogram to expose "for the light" side of the graph, not allowing blowout (clipping) of light detail, and then you ignore the left side of the histogram graph, allowing the dark values to fall where they may, you will achieve a look very similar to a well exposed Fuji Provia® slide. Then, if you take the image into your computer and boost the saturation of the colors a bit...you now have achieved a super-saturated Fuji Velvia® look.
Your histogram is a very valuable tool, and this article just touches on its capabilities. There are many discussions out there about how the histogram represents a combination of the three RGB channels (Red-Green-Blue), or only the green channel of the three in the RGB, or even only a combined luminance channel. It varies according to the camera. Your manual will tell you what type of histogram your camera has, if you are interested.
The manipulation of the histogram "levels" in Photoshop® is a detailed study in itself. Learn to use your computer to tweak your images, and you will be able to produce superior results most of the time. But, first 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...
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