Histograms

A histogram ,in photography, is a graphical representation of the tonal distribution in a digital image. The horizontal axis represents the different tones, whereas the vertical axis represents the number of pixels at each tone. It is a standard 8-bit scale of 256 levels, and begins from 0 (black) on the left to 255(white) on the right. By looking at the histogram for a specific image a viewer can judge the entire tonal distribution at a glance and will be possible to tell if any image detail has been lost due to blown-out highlights or blacked-out shadows. Modern digital cameras are able to display a histogram on their screen which can be very useful to photographers as it accurately shows the distribution of tones captured.

Highlight clipping is when image data is lost due to excessive brightness; no detail will be visible in the clipped areas, only pure white will be seen. Shadow clipping on the other hand is when image data is lost do to the lack of any light (complete darkness); no detail will be visible in the clipped areas, only pitch black will be seen.

I found a great site to reference to (http://www.cambridgeincolour.com © Sean Mchugh) where I found the diagram below that clearly shows the different parts of a histogram and what each section represents.

[1] © Sean Mchugh

Low Contrast Images

-1 stop

correct exposure

+1 stop

A low contrast photograph is one that appears flat and has a very small range of tones. My low contrast image is that of a snowy patch of field with the footprints of an animal crossing it. On the histogram the values are squeezed together quite tightly due to the lack of different tones. With the snow being white, the histogram for the correctly exposed image is plotted on the right hand side of the scale because this is where the light tones are. As I increased the exposure by 1 stop, the histogram moved further to the right as the image became brighter. Under exposing the shot by 1 stop moved the histogram to the left because the pixels became darker.

Highlight Clipped Areas

Due to the over exposure of the +1 stop photo, image data has been lost where the sun is hitting the snow (marked in red). The white is too bright for the camera to capture and therefore goes off the scale of the histogram.

Average Contrast Images

-1 stop

correct exposure

+1 stop

A photograph of average contrast is one that appears to contain a lot of different tones so that the image appears to have some depth to it. My average contrast shot is a photograph of some clouds during sun set. At first I found it quite difficult to decide what an averagely contrasting image would look like but took this shot an felt it fit the part although it does border on being low contrast. On the histogram, the values are nicely spread over the tonal range but do not spread to either end of the scale. The histogram for the correctly exposed shot is plotted in the centre of the scale and as the exposure is decreased by 1 stop it moves to the left due to the increase in dark tones. As the exposure is increased by one stop, the histogram moves to the right due to the increase in light tones.

There aren’t any particularly bright or dark areas in this shot and so there hasn’t been any highlight or shadow clipping.

High Contrast Images

-1 stop

correct exposure

+1 stop

A photograph of high contrast is one that has areas of very bright colours and areas of very dark colours. My high contrast shot is a photograph of a sunset across a snowy farmers field. On the histogram, the values are spread over the entire tonal range with bright and dark tones being the most abundant in the correctly exposed shot. As the exposure was decreased by 1 stop the number of dark pixels increased and bright pixels decreased making the histogram high on the left and low on the right. As the exposure was increased by one stop, the opposite occurred; the number of bright pixels increased and dark pixels decreased making the histogram high on the right and low on the left.

Highlight Clipped Areas

The sun is so bright that in all three of the high contrast shots, there was some highlight clipping. The underexposed shot had the most amount of clipping where as the overexposed had the least. There wasn’t any shadow clipping but I feel this was due to the overall brightness of the scene. The snow will have reflected the sun’s light onto the dark areas, meaning that my camera could capture more of their details.