A histogram is a graph which illustrates the tonal range of the pixels from which the image is composed. The vertical axis displays the number of pixels at the brightness level and the horizontal access From left to right the histogram displays the brightness level as shadows, midtones and highlights.
By understanding how to read a histogram, the photographer can decide if the image can be processed correctly.
There are three main types of histogram – low contrast image, average contrast image and high contrast image. I selected 3 differing contrasts to enable me to study each histogram in more detail.
I knew I was able to review my histograms once I had taken the photograph both on the camera and in curves and levels in Photoshop . I discovered that live view can be selected to display the histogram before the image is taken. This could be useful to give an indication of whether the scene was correctly exposed.
High Contrast
Under exposed
1/45 F11 ISO100 65mm –1exp comp
With high contrast and underexposed, I expected there to be a high number of pixels in the dark area of the graph (left hand side). An exposure compensation of –1 will make the shadows appear darker. I think there is still detail in the shadows, although the histogram would appear to suggest that over half of the image contains dark areas.
Average exposure
1/45 F11 ISO100 65mm 0 exp comp
Freeman (2010)suggests that a similar looking histogram “contains several tonal groups , but evenly spread from shadow to highlight.” I would agree that my histogram which contains the same characteristics was even in tonal groups. The shadows are dark and still show detail in them and the highlights are bright although there is a spike on the far right which indicates that the highlights are burnt out.
Overexposed
1/45 F11 ISO100 65mm +1exp comp
With this histogram, the image is noticeably overexposed which is displayed on the histogram because there are more pixels in the highlights area, including a large spike at the far right hand side. When looking at the image, one can see the shadows are poorly defined which shows on the histogram as the area on the left containing the pixels in shadow counts for about a quarter of the image.
Average Contrast
Under exposed
1/125 F4 ISO100 82mm –1 exposure compensation
This histogram shows the pixels as being concentrated in the shadows area with not many highlights. Comparing this with the image would suggest to me that this is what the camera saw. There are several dark, shadowy areas within the image. A longer shutter speed would enable more light to reach the sensor resulting in more pixels being in the centre (good tonal range) and highlights.
Average exposure
1/125 F4 ISO100 82mm 0 exposure compensation
This image suggests the exposure is about average. It is not quite the perfect shape there are not quite enough highlights shown to make the shape symmetrical. There is slight shadow clipping on the left of the histogram, maybe from the dark branches of the tree.
Over exposed
1/30 F4 ISO100 82mm +1 exposure compensation
With the overexposed version, the pixels are in the highlight area of the graph.There is some highlight clipping. I would expect this because the exposure time was lengthy with a large aperture, enabling more light to reach the camera’s sensor.
Low contrast
Under exposed
1/125 F4 ISO400 65mm –1 exposure compensation
With low contrast and under exposed, I would expect the pixels to be highest on the left hand side of the graph (dark area). The image looks fairly dark with very few highlights displayed. A warning appeared when first opened in Photoshop because the shadows reached the top of the histogram showing pixels at the top of the graph.
Average Exposure
1/60 F4 ISO400 65mm 0 exposure compensation
This average exposure shows that the image still contains several dark toned pixels and less highlights. I expected this type of histogram because the signpost was in the shade on a cloudy day resulting in not much contrast. Even with this histogram, the shadows reached the top of the histogram and a warning appeared in Photoshop.
Over exposed
1/30 F4 ISO400 65mm +1 exposure compensation
These pixels are in the area of good exposed tonal range in the middle and still several towards the right – the area of highlight. There is some highlight clipping which is shown by the spike on the histogram, indicating that the amount of pixels in this section is very high. The length of exposure and large aperture enables too much light to enter the camera, resulting in overexposure which is displayed on the histogram.
Reference
Freeman (2010), p19, Mastering Digital Photography, Ilex Press, Lewes, UK
Bibliography
http://www.photoshopessentials.com/photo-editing/histogram/ accessed 22/6/13
http://www.digitalcameraworld.com/2012/04/18/how-to-read-a-histogram-photography-cheat-sheet/ accessed 22/6/13
Freeman (2010), Mastering Digital Photography, Ilex Press, Lewes, UK
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