Most photographers contend with reviewing images on the back of the LCD screen to assess how well exposed they are. But because the LCD screens can be rather small and well illuminated, they can make photos appear better than what they actually look like on much larger computer screens.
Furthermore, sometimes viewing photos on the camera’s LCD screen might not show a loss of detail in the shadows or dark areas of an image, nor in the white or bright areas. When we’re really concerned about getting the best exposure for our photos, we might use the camera’s zoom-in feature to assess exposure and sharpness of our images. Additionally, we could use the Histogram tool of both our digital camera and image editor to see how well exposed our images are.
To some novice shooters, the Histogram may look like something a technician would use to see if your camera’s functioning properly. While the Histogram feature can be found in all 35mm digital cameras, the tool might not be easily visible; with some cameras you may have to push a button or two to bring it up. But that’s okay, once you learn how to interpret the Histogram, you might find it worth accessing and using a regular basis.
For this tutorial we’re going to primarily in Photoshop to understand how it works. The Histogram in a digital camera works nearly the same way it does in an image editor like Photoshop, so this tutorial also details how to read camera histogram.
What is the Histogram?
The Histogram is basically a graph showing the brightness distribution of an image with pure black on one end, pure white on the other and grey in the middle. Because measuring light is what photography is about, the Histogram works pretty well in a digital camera, but it also may be a little confusing, so I’ll try to clarify with a few image examples.
LIGHT RANGE
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.
Look at Figure 1 below. The gray rectangular area is a representation of an in-camera histogram. Examine it carefully! Think about it for a minute before reading on.
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.Over-exposed Images
Many images we capture might tend to be over or under-exposed. It’s easy to tell when a shot is way overexposed because most of the detail in the image in blown out. In scenic images, a loss of detail often occurs with clouds and the sky in the background, such as with photo below. If you expose for the buildings, the background sky gets overexposed.
In the Histogram, the graph shows a bias to the right end. When the graph is bunched up against the right side (pure white), it indicates the areas of the image that are overexposed, or clipped—a technical term meaning a loss of detail or information in an image.
Underexposed Images
When the Histogram is bunched up toward the left (pure black), it indicates that areas of the photo are underexposed, and that details are clipped in dark and shadow areas.
Balanced Image
A more balance Histogram depicts a graph with the pixel representations spread across the graph and tapering off on both ends, such as in this image.
However, it’s often difficult to achieve a balanced graph for most photos. Significant contrast in the light, dark, and mid-tones in the foreground and background areas of a photo typically makes it difficult to get a balanced exposure.
In the first photo example above, it’s nearly impossible to expose for both the background and the foreground of the image. So one solution is to meter for the sky, which means under-exposing the photo to capture the details of the sky, and then open up the shadow areas of the foreground in an image editor.
Sometimes the Histogram may show exposure clippings that are not a particular problem in the photo. In the image below, the graphs depicts a bright spot in the corner of the photo and some loss of detail in the dark areas of the flowers. But these are not major problems for this image. The details in the foreground are pretty much where you want them to be.
By the same token, in this photo below, the light on the subjects was even, and the Histogram depicts an exposure with no clippings. And because the graph is spread out and not bunched up in the middle, it indicates that the photo also includes a nice contrast.
Again, the Histogram results don’t have to be perfect, but when you see clippings on either side of the graph, you will want to review your images to see if there’s a loss of detail in important areas of the image. The Histogram is also useful when it’s difficult to review your photos such as in bright sunlight on the LCD screen. The Histogram can give you pretty good indication about the accuracy of your exposure.
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