Understand Histograms to get a better idea about Exposure

 


What is a histogram?

A histogram is a ‘diagram in which columns represent frequencies of various ranges of values of a quantity’, says The Concise Oxford Dictionary. This is one of those definitions that only makes sense once you have seen examples.

In statistics, histograms are used to make numbers visual. A simple diagram is usually much easier to understand than a mass of figures. Also, diagrams can reveal relationships between some of the numbers that are hidden in the raw data.


Many of the features of digital cameras are similar to those of film cameras, or can be related to film photography. Histograms are one of the exceptions. You might have come across them if you are a statistician, but histograms only entered the world of photography with digital imaging.

Many digital cameras include a special graph called a histogram that shows the brightness levels of an image ranging from pure black on the left to pure white on the right. It can give you excellent information that helps you better adjust your exposure.

There is no such thing as a perfect shape to a histogram. It can reflect only the real-world brightness levels of your scene. Although the left side can be important, the key to reading a histogram is to watch the right side. If the histogram slams up against the right side so that its slope is abruptly chopped off, it is said to be clipped, and this represents an image with lost or blown-out highlight detail.

A gap on the right side of the histogram is a problem, too. It means your photo is underexposed and you are not fully using your sensor. In general, adjust your exposure so that highlight detail is retained without clipping and the histogram is not crammed to the right side or showing a big gap there. 


Many digital cameras enable you to change the brightness of the LCD screen used to view images that you are about to take or have taken. Changing the brightness level or viewing the screen in bright light can cause you to misread the exposure. If your camera offers a histogram, you can use it to give you an accurate view of the exposure, regardless of the LCD screen brightness setting or bright light. However, it is not compulsory to always check the histogram if you have set the lcd brightness in the middle and can judge the exposure by viewing. Keep in mind that when viewed outdoors, the image appears to be dark but it is not all that dark.


Digital photo editors, such as Photoshop Elements, have a feature similar to the histogram on some digital cameras for reading the overall brightness of an image. The Levels command provides a histogram along with the ability to modify the tonal range and overall image contrast. 


256 tones

In digital photography, a histogram displays the tonal distribution of an image. It does this by categorising every pixel into one of 256 groups, where 0 is black, 255 is white and all the other numbers represent shades of grey.

On the histogram, the horizontal axis shows the group numbers with 0 (black) at the left and 255 (white at the right. The vertical axis indicates the number of pixels in each group.
 

Within these axes is the impression of a solid graph − in fact, what you are seeing is 256 upright lines crammed so close together that they are touching each other.

What does a histogram tell you? Well, if nothing else, it will warn you of over or underexposure. If there are lines right up against the left of the diagram, the chances are the image is underexposed. If there are lines right up against the right of the diagram, the image is probably overexposed. A typical well-exposed image will show the main distribution of tones around the centre, reducing towards the left and right (though there will be exceptions).

EOS digital cameras, except the DCS series, will show a histogram for each image you shoot. When an image is displayed on the preview screen, press the ‘info’ button to the left of the screen. The display will change to give you a smaller image, plus the histogram and some basic exposure data.

An additional feature is the ‘highlight alert’. This makes any overexposed areas within the image ‘blink’. This can be very useful when shooting static subjects, such as landscapes. If parts of the image blink, you can apply exposure compensation and take another shot to bring the tones into an acceptable range. Use this feature and you should never come away with another overexposed image.


Software

It is not just on the backs of the digital cameras where you will come across histograms. You will also find them in some imaging software. The Canon File Viewer Utility shows them, for example, as does Digital Photo Professional. But the place many photographers will come across histograms is in Photoshop (Image>Histogram). Not only can you see the overall display, but you can also check the tones in each of the 256 levels for the overall image, or for each of the red, green and blue layers.

You will also find histograms in some of the other areas, such as Image>Adjust>Levels, where you can adjust the image to suit its output by clipping the levels so that you work with fewer than 256.