This is the second post in this “How to craft better photos” Series. I will be talking about the basic concepts of photography exposure histogram.  Follow this link to read the first article titled Understanding Exposure Will Help to Improve Your Photo.

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                                                              Taking proper exposed images matter!

Using the histogram in you camera’s image viewing LCD will help you to take a much higher number of well exposed images.  I will cover this feature with enough detail to give you a working knowledge of how to use the histogram to make better pictures.   I do not pretend to cover everything on histogram but I will present enough knowledge to improve your proper exposure technique immediately.

Human Eye and the Camera Sensor

The human eye can cope with an enormous brightness range.  For example on a bright day we can see in to the deep shadows cast by a tree at the same time we can see details in the lightest part of a white cloud directly over the tree and lit by the sun.

The camera’s sensor can only capture a more limited brightness range in a single exposure.  If you want to record the subtle detail in the lightest part of the cloud then detail will be lost in the shadow areas of the image. Similarly if you set exposure to reveal full detail of subjects in shade then you will end up with "washed out" highlight detail where light tones become featureless white.

To make easier the understanding of this, I will use the term “f-stops” to represent the quantity of light  that can be capture.  The human eye retina has an static contrast ratio of around 100:1 that is about 8 1/2 f-stops. Today’s digital cameras are capable of  “see” about  5 or so usable f-stops of light.  Unfortunately, many of the higher contract subjects we shoot can contain up to 12 f-stops of light values, this is more than it is possible to capture with a digital sensor today.  That is why it is important to understand how your digital camera records light, so that you can better control the quality of the captured image.

What is the histogram

Histogram is a topic that we could spend a lot of time talking about but I will give you an answer that will help you to understand it better, the simple one.

The histogram is a graphical representation of the exposure of an image.  Exposure refers to the amount of light that strikes the camera’s sensor to form the photography image.  Therefore, the histogram is a representation of how much light and tones the image has.

The image at the left is a typical histogram.  I will explain the meaning of its part.  Click here to have a better view of it while a explain.

The graph has two axes: across the bottom (the x axis) go all the distinct values your data can have, and up the side (the y axis) you plot how many times each value occurs. In a digital camera, values in 8-bit mode range from 0 through 255 for x axis. (0 = Pure Black, and 255 = Pure White).
The subject brightness range we can capture in any given image and still retain detail is about five f-stops as I said before. We can therefore divide a scale of image brightness into fifths and consider each one to be about a stop. These divisions can be referred to as very dark, dark, medium, bright, or very bright. This becomes the x axis of our histogram.

On the left hand end is black (zero) and on the right is white (255), with every other possible brightness spread in between. Eighteen percent medium gray would be dead center.

If we count how many pixels within our image have each distinct brightness value and plot this on the y axis, we get our histogram.  The top of the histogram (top of mountain peak, y axis values) represents the number of pixels of color in that x range, 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)

How interpret a the histogram

A “correct” exposure can be regarded as one where the most important areas of the image looks “right” with its details.  Therefore, there is not a perfect exposure histogram.  
A dark, low-key image will have a histogram that clusters mainly around the left end. The histogram of a bright, high-key image will tend towards the right. The one form an image with a more "average" character will likely be spread more evenly across the spectrum ( as a mountain).

	Underexposed Image Histogram This histogram is clipped off on the left (dark) side which means that the image is generally too dark .  In fact , the light range in this histogram shows that exceeds the capability of the camera’s sensor which means that shadows details are lost.
	“Correct” Exposed Image Histogram This histogram contains almost a full range of light that the camera’s digital sensor can record.  The light range is represented by a graph like a mountain peak.  This is a “correct”exposure, since the histogram is not clipped off to the left or the right.  If you can make your image histogram look like this, you will have good images.
	Overexposed Image Histogram This histogram like the one at the left is clipped off on the right side of the histogram  (white area) which means that the image is generally too white .  In fact , the light range in this histogram shows that exceeds the capability of the camera’s sensor which means that highlight details are missed.

Here are three different exposures of the same image and their corresponding histograms.

The underexposed one is starting to clip on the shadow end which may or may not be an issue.  It depends on what we want. Deep shadows and silhouettes are supposed to come out this way. Given the subject matter though, not having any pixels over medium would definitely be considered a problem.

The histogram for correctly exposed one shows a subject clustered near medium, with no completely black or white pixels (no clipped to the left or the right).

The overexposed version clearly shows clipping in the highlights. We have lost data here that can not be recovered later since we never captured it in the first place when the image was shot.  Unlike on the shadow end, burned out white is rarely desirable.

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.

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.  This technique is commonly referred to as "exposing to the right" and can work on all types of images, providing we adjust the exposure after the fact after uploading these images to our computer.

Homework - Using the histogram While Shooting

So now you know what a histogram is – grab your digital camera’s manual and work out how to switch it on in playback mode. This will enable you to see both the picture and the histogram when reviewing shots after taking them.

Keep an eye out for histograms with dramatic spikes to the extreme ends of either side of the spectrum. This indicates that you have a lot of pixels that are either pure black or pure white. While this might be what you’re after remember that those sections of the image probably have very little detail – this is a hint that your image could be either over or under exposed.

Histograms are the best way to assess the accuracy of an exposure because they represent the exposure in a graphical form that is unaffected by viewing conditions.

Hopefully this will give you some understanding of how to read the histogram for an image. Clearly the best way to learn what a histogram can tell you though is to go out and shoot some images of your own, paying attention to the histograms after each shot.  When you know how to interpret a histogram you have a much better method to check exposure.

I will love to see comments about your experiences…    You may share them with us in the comment area.

Read the Full Series

This post is part of a series on “How to craft better photos” . It will be all the more powerful if you taken in context of the full series which looks at 5 points of how to improve your photography and do it in a creative way.  Start reading this series here.

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