The normal distribution of tones in a digital image produces a smooth histogram with every one of the 256 tonal levels occupied. In a correctly exposed image, the peak of the graph is in the centre and the graph tapers down at each end to the 0 and 255 points.

 

The normal distribution of tones in a digital image produces a smooth histogram with every one of the 256 tonal levels occupied. In a correctly exposed image, the peak of the graph is in the centre and the graph tapers down at each end to the 0 and 255 points.

While this will produce nice-looking prints from most snapshots, it isn’t necessarily what we require for all types of images. The shot below was taken with the idea of producing a silhouette of a crowd of people against a colourful sky. Consequently, although the majority of tones are clustered around the mid-point in the graph, there are enough very dark tones to produce a peak on the left side.

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We can emphasise the silhouette effect – and, at the same time, deepen the colours of the sky – by moving the white slider in to the end of the graph and the black slider to the end of the mid section of the graph.

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Performing this action on a JPEG file cuts roughly one third of the pixels out of the image and discards them. Because JPEG images only cover 256 tonal levels, the result is a ‘combed’ histogram in which the gaps between the black lines and white spaces (the ‘teeth’ of the comb) represent missing tonal values.

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When these gaps become too wide, tones begin to ‘drop out’ of the picture and it becomes ‘posterised’, as shown in the illustration below.

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Moving the three sliders to the centre of the graph ‘drops out’ two thirds of the image tones, leaving the posterised image shown below.

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The only way to avoid this problem is to shoot raw files. Although this is only possible with some advanced digicams but all DSLRs support raw file capture. The raw files are then converted into 16-bit TIFF files with either the software provided with the camera or a third-party converter like Adobe Camera Raw (which is available as a free download to users of Photoshop Elements or Photoshop).

Whereas an 8-bit JPEG image contains 256 tonal levels, 16-bit image file contains 65,536 levels of image data. Admittedly, the file is much larger; but, if you need to focus down on a limited range of tones, the scope for doing so is dramatically improved.
Compare the histogram below with the combed histogram from the JPEG file on the previous page.

Both files were given the same degree of Levels adjustment; yet only the TIFF file retains a smooth histogram without gaps.

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The ability to work with more image data provides the serious photographer with the ability to manipulate images with a high degree of precision and subtlety – without risking tone ‘drop-outs’. This is the main reason serious photographers shoot raw files instead of relying on JPEGs.
This is an excerpt from Post Capture Pocket Guide.
Click here for more details on this and other titles in the Pocket Guide series.

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