The purpose of this exercise is to increase your familiarity with histograms by relating each one to the image you have just shot. This is intended as an aid to recognising the most basic characteristics of an image. Histograms appear twice in a normal workflow- the first time on the camera’s LCD screen, the second when the image is being processed- and it is valuable to make use of both occasions.
Find and shoot the most basic categories of scene by contrast. These are low-contrast , average contrast, and high contrast. For each of these shoot not only an averagely exposed version, but also one that is approximately one f-stop darker and another that is approximately one f-stop brighter. As you shoot, make a point of checking the histogram on the camera’s LCD screen.
Now open and examine each image in Photoshop, and make sure that the histogram is displayed. Make a screenshot of each histogram and write a short note for each with your explanation of how they relate and how they vary with exposure.
Prior to this exercise I had no idea what a histogram was, and had never paid particular attention to it on my camera. Therefore, before beginning this exercise, I read Michael Freeman’s book entitled, ‘Top Digital Photography Tips’ which gave some great descriptions of histograms and how to use them.
“Histograms are a very useful tool that many cameras offer their users to help them get a quick summary of the tonal range present in any given image. It graphs the tones in your image from black (on the left) to white (on the right). The higher the graph at any given point the more pixels of that tone that are present in an image.” (1)
“Histograms may not at first look like a sympathetic way of judging images, but with experience, they can become a fast track assessment of exposure, brightness and contrast.”(2)
On my Canon 650D my histogram display is enabled by clicking on the info button whilst in image playback.
I used my camera’s exposure metering scale to get images one stop under/over exposed.
High Contrast Image- Average Exposure. The dark trees against the bright sunlit sky form a silhouette filled image, providing a high contrast of dark and light tones throughout the photo. A few highlight warnings appeared between the trees in the whiter parts of the image. The graph is very well spread out from one end of the overall range to the other, with high peaks at each end- showing the distinct high levels of dark and light tones in the image.
High Contrast Image- Overexposed. There were further warnings to signify that the image was beyond my camera’s dynamic range. The warnings were much more widespread than the average exposed shot, due to the overexposed qualities of the photo which made for many more light tones. Increasing exposure also moves the dark tones to the right slightly and seems to flatten the extreme brightness levels to the right.
High Contrast Image- Underexposed. The highlight warnings were reduced in this shot due to the darker exposure which helped to lessen the extreme brightness of the sky. At decreased exposure, the levels move slightly closer to the extremes.
Average Contrast Image- Average Exposure. In this average contrast image the histogram seems to cover a wide range but stays away from the extremes of brightness\darkness.
Average Contrast Image- Overexposed. Again there is a spread of information all across the graph. Increasing the exposure basically moved the graph slightly to the right.
Average Contrast Image- Underexposed. I can see that the amount of pixels on the right side of the graph is now considerably less. Due to underexposing the image there are fewer light values in the image, therefore the graph reflects this as it bunches up to the left.
Low Contrast- Average Exposure. In my low contrast image at normal exposure the histogram occupies just over two thirds of the complete width of the graph. In this image there isn’t a great deal of variation in tones and this is shown in the histogram with a fairly central peak that trails off rather steeply at either side.
Low Contrast- Overexposed. By increasing the exposure the graph has shifted to the right compared with the normal exposure. Also, although the values occupy roughly the same amount of the complete range as the normal exposure there has been a shift in values with more values now on the right side of the already used area than the left side.
Low Contrast- Underexposed. By reducing the exposure, the graph has moved slightly to the left compared with the normal exposure. The values are now much more bunched together and therefore occupy much less of the complete range of the graph.
This exercise has given me a good insight into what a histogram is, and I have learnt the basics of how to read an image through it.
(1) Rowse, Darren, http://digital-photography-school.com/understanding-histograms
(2) Freeman, Michael, Michael Freeman’s Top Digital Photography Tips (2004)