Reading Image Histogram is essential in understanding the distribution of the photograph tone. Image histogram graph can be used to evaluate the tonal balance of the photographs.
It’s common to evaluate the level of photography exposure of the photo result by reviewing the images directly via the camera LCD screen or via the printing result. The other technical way to evaluate the tonal balance of digital image is by reading image histogram. Some digital cameras have been equipped with this tool either live histogram prior pressing the shutter button or reading the histogram graph of the images stored in the camera memory card. Some image editing tools such as Adobe Photoshop or complementary software that come with the camera such as ZoomBrawser EX that comes with Canon camera; can also be used in the computer.
The figure below is a simple histogram graph of a photo image. When the graph shows the peak at the left side of the graph mostly – the image is predominated by the dark tone. While if the image histogram shows the peak mostly dragged to the right side – the photo image is predominated by the bright tone. The ideal is the mid-tone when the histogram graph is predominantly occupying in the middle of the chart. The range between the dark tone and the bright tone that the digital camera can handle is called the Dynamic Range of the digital camera which is typically 4EV.
The following discussion will explain more detail about reading image histogram using the software utility which represents the same histogram graph of the photo images in the camera.
Hostogram Graph - 4EV Dynamic range
By reviewing histogram graph; the photo image can be evaluated to see if they tend to be bright or dark; the tone range still complete or as a whole how bright and dark the photo image is. Basically image histogram is a bar chart that shows how many pixels for each brightness value on the whole tonal scale from completely dark black to totally white.
Each photographs has different image histogram pattern depends on its tone balance. The ideal is zero level on each edge chart either on the right side or on the left side.
A Chopper with cloudy background
The example of the above photo; the histogram graph pattern is predominantly occupied by nearly bright cloudy on the sky and few part with dark area.
The histogram graph of the above photo
Various Histogram Patterns
A common image histogram pattern has zero curve at both left and right sides of the curve; but higher to maximum at the middle of the curve. But some photographs have different histogram graph pattern. When a shade area of the subject is captured in a situation where the sky is bright and clear; the image histogram will show both peaks at the right and the left chart and a flat pattern at the middle.
The image histogram shows both peaks at the right and the left chart
There is eventually nothing wrong with the exposure; it is just a characteristic of the photograph – a backlight background that creates a silhouette image.
Check also the article about tricking the photography background.
Flat Tone
In a situation when there is not much contrast on a subject; the image histogram curve will show flat to zero nearly along before the left and the right edges of the histogram graph. This is the characteristic of the flat tone image with no contrast. There is no area with high brightness or area with significant darkness. Whereas, this photo usually needs a full tone range to show the depth and the rich of its color.
Av=1/80s; ISO speed=200, f/4 Canon EF 70-200 IS L
Underexposure Histogram
Utilizing image histogram can help photographers review if the photo result is underexposure or not. The photograph below is completely underexposure and the histogram graph shows the curve drawn to the left side significantly. There is no brightness area at all as shown by the image histogram that there is no curve pattern at the right side.
Check also how to trick the digital camera exposure.
Underexposure photo - the curve is drawn to the left side significantly
But some intrinsic dark or black subject can also contribute the underexposure pattern on the histogram chart. Have a look at the photo below; the dancers are wearing black singlet that contribute the left side histogram graph pattern; even though as a whole the photograph is not underexposure.
intrinsic dark or black subject contribute the underexposure pattern on the histogram chart
Bright Histogram
In this photograph; the object was intentionally captured with a bright background, the grey cloudy sky. And the image histogram pattern can be guessed; the curve is brought to the right side. It is contrary with the underexposure pattern; the bright background contributes the right curve – a completely bright pattern.
Broght background - the curve is brought to the right side
So image histogram is like a diagnose tool to show how the tone is distributed on the photograph. Histogram graph doesn’t show you how the photographs should look like – it’s your job as the photographer.
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