A histogram is a graphical representation of the pixels exposed in your image. The left side of the

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In this article we’re going to look at how to read it and use it to your advantage to help you do just that. Getting the best exposure (there is not such thing as the “correct” exposure, as it’s all subjective) in camera should be your goal every time you click the shutter. Using these tips should help you increase your success rate. What is a histogram?
Dictionary definition: A bar graph of a frequency distribution ü in which the widths of the bars are proportional to the classes into which the variable has been divided and the heights of the bars are proportional to the class frequencies.
A histogram is a graphical representation of the pixels exposed in your image. The left side of the graph represents the blacks or shadows, the right side represents ü the highlights or bright areas and the middle section is mid-tones (middle or 18% grey). How high the peaks reach represent the number of pixels in that particular tone. Each tone from 0-255 (o being black and 255 being white) is one pixel wide on the graph, so imagine the histogram as a bar graph all squished together with no spaces ü between each bar. Have a look at the diagrams below:
We can tell an image is well exposed if it reaches fully from edge to edge without a space on one side of the graph, and isn t heavily going up one side or the other. In an ideal world, it should just touch the left and right edges, and not spill up the sides, with a nice arch up in the center. However that doesn’t always apply in every situation, for every scene. Here are a few examples:
This is a histogram for a light subject (white cat) with mostly light tones in the scene and few dark areas. See how it is shifted to the right now versus the dark subject. This is correct. If you change your exposure on this to make it in the middle you will have grey cat and not a white one. When the histogram tells you to adjust your exposure
Gaps on either end indicate you are missing information and your exposure can be shifted safely without losing detail. When your graph is shifted too far in one direction or the other so that it does not even touch the other edge – that means you can safely ü shift your exposure to cover more of the range of tones. Let’s look!
This graph shows an overexposed image, notice the gap on the left side indicating a lack of any blacks represented. It also means you will lose lots of detail in the white areas that may not be recoverable. In this case shift to give your image less exposure and shoot the scene again.
This one shows the opposite. Now we see a gap on the right side of the graph indicating there are no whites represented so the image will be dark, too dark. You can safely give the image more exposure until you see the graph just touch the right edge of the graph. What do the spikes up the sides mean?
Spikes up the left or right edge indicate “clipping” ü of that tone and loss of detail in that area. Clipped areas are often unrecoverable, especially in the highlight area but it is generally advised to expose so you your graph just touches the right edge and keep your highlight details. It is usually easier to recover some shadow ü detail and retain a decent image, than try and create highlight detail that isn’t there on the file.
In some scenes, ü however, it may not be possible to keep the graph within an acceptable range. For example, if you are photographing a scene with extreme contrasts such as: a sunset; bright sunlight and deep shadows; or an inside a building ü where you show outside the windows as well. In all of those cases you will not be able to keep from clipping either your blacks, or whites, or even both.
No it’s not wrong. You can’t really “correct” for it but you do have a decision to make when you see something like this. Do you shift the graph left and maintain highlight detail, or shift it right and keep shadow detail?
There is no right or wrong here, it’s how you interpret the scene before you. If in doubt, shoot both and decide later. The graph above comes from the image below, so as you can see it is not the incorrect exposure at all.
Using advanced techniques like image merge/blend, HDR and processing in Lightroom 4 (or PS CS6) you can compress the contrast ü range of the scene to fit within the histogram and therefore have details in all areas.
In the image above, I’ve used 4 bracketed images (taken 2 stops apart), and the HDR tone mapping process to bring the dynamic range of the scene down within printable range. ü One more handy thing on your camera – the “blinkies”
To help you establish how far to go in the image bri