Finding your level

Some golden oldies this month: Photoshop's Levels Tool and Paintshop Pro's Histogram Adjustment, which can still show the youngsters a thing or two.

Sometimes the worst thing about "featuritis" - the condition caused by flashy but superfluous features being added to new versions of software - is the fact that very often, existing tools can do the job just as well.

Image-editing applications tend to be the worst offenders. Any brief demonstration of Photoshop's Shadow/Highlight feature, which brings detail to an image's lightest and darkest areas, inevitably has an audience cooing in appreciation. But few realise you can achieve a similar result with Photoshop's Levels or Paint Shop Pro's Histogram Adjustment dialogue boxes (see FIGURE 1).

In more than a decade of working in multimedia, I remain amazed at the proportion of people who remain oblivious and indifferent to their purpose. Yes, Levels and Histogram Adjustment can be off-putting to beginners with their complex-looking graphs but, when it comes to quick image adjustment, there's nothing better. It's the correction I use in virtually every image I tweak for Web or print use.

The histogram gives an overview of a picture's tonal range - its contrast and colour balance - by graphically displaying the number of pixels at each colour level, from darkest black to white. Using this histogram you can see accurately how well a picture is reproducing shadows, highlights and mid-tones, and change those settings to improve contrast and tonal range.

Understanding a histogram is simple. If most pixels bunch to­wards the left, the image will look underexposed, while an overexposed photo will have a high tonal range, with pixels congregating towards the right. By contrast, an image with full tonal range - usually your objective - has a number of pixels in all areas, albeit not necessarily evenly spread.

Lowering the tone

You can give almost any image a good tonal range. The important parts of the histogram are the triangular sliders immediately underneath it. If you move the left (black) slider inwards, anything to its left will be turned black. Similarly, moving the white (right) slider inwards turns anything to its right white.

To get the optimum tonal range, move each slider to the end of the histogram curve. This will distribute the pixels more evenly. The central slider, which controls the photo's midtones, can then be dragged to the left or right to lighten or darken the image as a whole. The result should be a much more balanced picture.

It's important to emphasise the value of working on layers when adjust­ing images. Every tweak - particularly to something as critical as colour balance - degrades it slightly.

Degradation can be easily seen in a histogram: the more gaps there are in a curve, the more degraded it is. And the more gaps, the more posterisation - abrupt changes from one tone to another - particularly in areas of flat colour such as sky.

Degrade expectations

This is one reason I don't use Photoshop's Auto Levels command, which automatically balances the tonal range. Its results are good, but it seems to severely degrade the image more than manual tweaks.

By working on layers, you lessen the impact of degradation and you can get back to the original, untouched image easily. You can either duplicate the original background image to a new layer and work on that or, in the case of Photoshop or Elements, adjust your tones on a special adjustment layer - choose Layer-New Adjustment Layer-Levels. Paint Shop Pro also has adjustment layers, although not for Histogram Adjustment.

Histograms are a feature of most image-editing applications - I use a near-identical feature in Ulead's PhotoImpact - and you can even apply the same techniques to taking pictures.

Most digital cameras have a histogram preview that shows an image's tonal range in the same way as an image-editing application.

While fiddling with the histogram values every time you take a family snapshot will not increase your popularity, for occasional use it's a good indicator of the type of exposure you'll end up with. As a guide, if you make sure the histogram peaks in the middle and isn't clipped at either end, you should get a decent exposure and tonal range (see FIGURE 3a and FIGURE 3b).

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Tom Gorham

PC Advisor (UK)
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