Your Camera's Histogram & Exposure Compensation

Digital cameras have a fantastic tool for helping us evaluate our photographs' exposure after we take them - the histogram. Not only are we able to correct poor exposure for our next shot but the instant nature of this hands-on feedback is a wonderful teaching tool. If we pay attention, we can learn by doing and improve in skill and speed every time we go out to shoot.

NOTE: If you are comfortable with camera histograms and want to jump to a more rigorous discussion of histograms, check out this Sekonic website article. Look carefully, there are several menu items available. I found it an excellent and detailed resource. Sekonic is a company that makes a very popular line of high-end exposure meters. I relied on their website and its articles to refine my study of histograms as I prepared this blog entry.

Still with me? Want the basics? Here we go.

First, what is a histogram? It is a graph - a pictorial representation of information arranged into bars of varying heights. The bars represent categories of information. The height of each bar represents how many of that category were counted. The term "Frequency" is often used instead of "how many".

Breath! We won't need any math more complex than counting for this example. You can do this!

Here's an example histogram. Suppose you decide to raid your piggy bank and count the money you find there. Being the neat and tidy person you are, you use a histogram to represent the coins you count when you crack Piggy. Like this: alt text

Histogram Example for piggy bank cons

Histogram Example for piggy bank cons

Okay, that's a histogram, now what does it have to do with your camera and images?

For each photo you capture, you camera evaluates the brightness of each pixel on the sensor on a scale of 0 (black) to 255 (white). To create the histogram, the camera gathers a count of pixels at each of the 256 brightness values and stacks pixels of like value together. You see a column of 0's, a column of 1's, a column of 2's, ... a column of 256's and so on.

The figure below is a representation of a camera histogram. The categories run along the bottom from left to right ( 0 to 256). That's the X axis. The count (Frequency) runs along the left edge and starts a zero and runs upward to some number we don't care about. It's just big. That's the Y axis. What we do care about is that the X axis is divided into 256 categories that are identified by the numbers "0" through "255" (black through white).

We can use the histogram to evaluate the dark vs light areas in our image. We can see if the exposure was too dark (underexposed) - pixels piled up at the 0 end of the scale or too light (overexposed) - pixels piled up at the 255 end of the scale. To see detail in our photographs, we must capture a range of values. If a photograph is mostly black or mostly white, there can be no detail.

No two histograms will look alike and there is not a perfect shape for a histogram. When you point you camera at a subject, the histogram will represent the dark and light shades of the framed image captured by your sensor. If the histogram shows most values piled at the zero end, let in more light. If the histogram shows most values piled at the 255 end, reduce the light entering your camera.

An example camera histogram

An example camera histogram

In the example histogram above, we see there are substantial pixels piled up at both the dark (A) and light (B) ends but area (C) shows very few pixels. This tells us that the scene has a wide range of light and dark with some very dark areas and some very bright areas. We will lose detail at both ends of this image.

Before we go on, here's a challenge for you. Get your camera and your manual and find out how to view the histogram on your camera.

Next let's look at real image and it's histogram courtesy of Adobe Lightroom. These histograms are in color showing the pixels of the various colors captured.

f/3.0 at 1/100 sec

f/3.0 at 1/100 sec

f/3.0 at 1/100 sec

f/3.0 at 1/100 sec

This image is over exposed but it still contains both dark and light areas. The exposure was taken at f/3.0 and 1/100 sec. Notice the significant number of pixels at the bright end of the histogram and how there is little detail in the bright area of the tulip.

The next image was taken at f/3.0 and 1/200 sec. Look for the change in the image and histogram. The decreased exposure allows us to see some detail in the lightest areas. The histogram reflects the reduced exposure.

f/3.0 at 1/200 sec

f/3.0 at 1/200 sec

f/3.0 at 1/200 sec

f/3.0 at 1/200 sec

Finally, let's look at an image with a further reduces exposure captured at f/3.0 and 1/400 sec.

f/3.0 at 1/400 sec

f/3.0 at 1/400 sec

f/3.0 at 1/400 sec

f/3.0 at 1/400 sec

With this last image and histogram pair, you can see detail throughout the tulip and the reduced exposure is reflected in the histogram.

Now that you know how to read a histogram, what do you do with the information it yields? You adjust your camera settings to yield a more pleasing image. Here's how.

Preview your image (chimp) and ask yourself, "Do I have too much light or too little light?" In this case, we had too much light. By keeping the aperture at f/3.0 and reducing the shutter speed from 1/100 to 1/200 to 1/400 we reduced the exposure by two stops of light resulting in a more pleasing image. 

Ok, you say, but I'm shooting in A-aperture priority or S-shutter priority or even P-program mode. How do I tell the camera I need more or less light? Doesn't the camera set the exposure for me? Yes, but you can override the camera-calculated exposure a bit by using exposure compensation - that little button that has the +/- on it. For most cameras, turning the shutter speed wheel while continuing to depress this button will allow you to reduce (-) or increase (+) exposure.

Give it a try. Send me an email and image at info@4riversphoto.com and tell me of your test and success. I'll publish your image and credit with a follow-up to this post.

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