Histogram and Exposure Control

Digital photography provides us with a very interesting tool to estimate exposure, the histogram. It provides the photographer with detailed information about the image’s tonal distribution.

The histogram consists of a vertical (Y) and a horizontal (X) axis. For exposure, the position on the x-axis is relevant. It provides information about under or overexposure.

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Brittany – Photo with correct exposure using a histogram

An image histogram’s left side represents the black and dark areas and informs from left to right about the light and pure white areas.  The more data points to the right side of the histogram, the brighter the image. The amount of data points offers information about the number of pixels with a particular brightness value. Depending on the histogram‘s positioning, it is possible to discern an under or overexposure.

Many modern cameras provide a histogram based on a greyscale channel as well as histograms for individual colour channels (red, green, blue). Usually the greyscale depiction offers a good overview. However, if the colour of a critical image detail is dominant, it can be useful to use a colour channel. 

The histogram can usually be viewed on the camera’s display screen (all compact cameras and some newer SLR cameras are equipped with live view) before capturing an image. The display is either divided up into areas for the subject and the histogram or the histogram is displayed across the image. This can be disadvantageous when capturing an image but the advantage is that exposure can be determined very precisely in advance. On other cameras, the histogram cannot be checked until after the image has been captured, in the preview. In order to see the histogram it must be activated in the settings (see user’s manual if necessary). If need be, the image could be captured again after checking the histogram and setting the right exposure compensation. If the image cannot be captured again, the histogram’s brightness values can be useful for future images. The histogram also provides useful information for image post-processing.

The histogram can very occasionally be misleading. The camera only shows the histogram it has accessed. This is, however, restricted to the camera’s dynamic range. It can occur that there is a peak beyond this range. In this case, the histogram could suggest an optimal exposure as the histogram is usually assessed by ensuring that the right and left side touch the graph as little as possible. Because the peaks beyond the dynamic range are not visible, the histogram cannot provide any information about them. If this is the case, an additional over and underexposure warning that flashes if an image section is over or underexposed can be helpful.

How can I tell on my camera’s display screen if a photograph has turned out well in terms of exposure?
Below the interpretation of histograms is illustrated with example photos.

Example 1 – Correctly exposed photograph

Study the histogram – it stretches out across the entire width without touching the right or left side. Even without looking at the photograph, a balanced exposure is to be expected with this type of histogram.

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Bretagne - Richtig belichtetes Foto mit Histogramm

Captured image facts:

  • Exposure time: 1/16 sec
  • Diaphragm: f/10
  • ISO rate: 100
  • Focal length: 45mm

 

Example 2 – Overexposed photograph – Two exposure levels (+ 2 EV)

Study the histogram – it touches the edge of right side of the histogram and is practically chopped off. This type of histogram indicates overexposure. The image histogram shows the course of a histogram that falls away on the right hand side. The fact that the number of data points on the right side is very high indicates that large sections of the image were overexposed.

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Brittany – Photo with overexposure using a histogram

Captured image facts:

  • Exposure time: 1/4 sec
  • Diaphragm: f/10
  • ISO rate: 100
  • Focal length: 45mm

In the following illustration, the overexposed areas are highlighted in red. For these areas the only information registered is “white”. These areas cannot be corrected later on by computer. The fact that the number of data points of left side of the histogram is very high indicates that large sections of the image are underexposed.

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Brittany - Photo with overexposure using overexposure warning

 

Example 3 – Underexposed photograph – Two exposure levels (-2EV)

Study the histogram – it touches the left side of the histogram and is practically chopped off. This type of histogram indicates underexposure. The image histogram shows a histogram course that falls away on the left hand side. The fact that the number of data points on the left side is very high indicates that large sections of the image are underexposed.
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Brittany – Photo with underexposure using a histogram

  • Exposure time: 1/60 sec
  • Diaphragm: f/10
  • ISO rate: 100
  • Focal length: 45mm

In the following illustration, the underexposed areas are highlighted in blue. For these areas the only information registered is “black”. These areas cannot be corrected later on by computer.

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Brittany – Photo with underexposure using underexposure warning

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