Project: A Sequence of Actions, Histogram

What is a histogram? At its simplest, it is the graphical representation of the distribution of tones across an image. Cambridge in Colour (http://www.cambridgeincolour.com/tutorials/histograms1.htm) considers it one of most important tools in digital photography; “A histogram can tell you whether or not your image has been properly exposed, whether the lighting is harsh or flat, and what adjustments will work best.” It’s so useful it’s important to understand.

As a graphical representation, the horizontal axis shows the level of brightness, from 0 (digital black) on the level to 255 (digital pure white) on the right. The vertical axis shows the number of pixels at each level of brightness. So a picture with everything to the left would be overall quite dark, to the right it would be light.

This exercise required taking photos of average contrast with a spread of pixels from 0 to 255, low contrast with all the pixels packed close together on the brightness range, and high contrast where the spread of brightness exceeds the scale. Here are the results:

Average Contrast

Average Contrast 0 stopimage

This average contrast, average exposure image has a spread of tonal range that fits (almost) neatly into the histogram. There are spikes at various points along the way but the black peters out around 0 and the white at 255.

Average exposure

Average Contrast -1 stopimage

The histogram has a similar shape to the previous with similar spikes but it is shifted to the left. Note that the brightest (with the exception of a few pixels) is short of the extreme bright whilst there are more at the dark end.

One stop under-exposed

Average Contrast  1 stopimage

Opposite to the previous, the histogram is shunted to the right. There is a big spike right at the bright end which indicates over-exposure.

One stop over-exposed

Low Contrast

Low Contrast 0 stopimage

A drab, flat even toned image as can be seen from the histogram with all the pixels bunched up near the middle. There are a few darker pixels, these can be seen at the bottom of the picture.

Average exposure

Low Contrast -1 stopimage

Similar shaped histogram but pushed to the left, toward the darker tones.

One stop under-exposed

Low Contrast  1 stopimage

Similarly, this has the same shaped histogram but pushed to the right for a lighter toned picture.

One stop over-exposed

High Contrast

High Contrast 0 stop image

There is too much tonal range here to fit into the histogram, they range from below 0 at the black end to above 255 at the white end. This is also an example of a low-key image, most of the pixels are at the dark end and the image overall is dark. There is a small spike at the bright end which corresponds to the highlights on the glasses.

Average exposure

High Contrast -1 stopimage

Being under-exposed this image is even more low-key. But now there is more detail on top of the box and there are some nice highlights in the glasses. I think this is the nicest in the set.

One stop under-exposed

High Contrast  1 stopimage

Even over-exposed by one stop, the tonal range is still to great to avoid clipping at both ends. Note the loss of detail on the box.

One stop over-exposed

The histogram is an extremely useful guide to how a digital image is exposed and this exercise has only scratched the surface. The camera can also show a RGB histogram which shows how the tones of these three channels are distributed, Photoshop can also show histograms for the three colours. Photoshop uses a histogram to adjust levels so that you can get more contrasty punch to an image. The camera histogram can be referenced during shooting to check on lighting and exposure.

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