Graphics - Colour levels and histograms

Many photo imaging programs such as ActiveShare can perform these functions with the click of a button, but on occasions the results can leave a lot to be desired. In these cases, a fully-featured level/histogram tool is needed.

For several years, the Levels function in Photoshop has been the most effective option for adjusting colour levels. Paint Shop Pro has been closing the gap slowly, but it still needs some work. You don't have to fork over $1400 to get this Photoshop tool; the lightweight Photoshop LE provides many of the same features at a fraction of the price ($223 RRP). Trial versions of both Photoshop LE and Paint Shop Pro 7 are on this month's cover CD, but this column will focus on using Photoshop and Photoshop LE.

Histograms and Levels can be a little complex, so don't be too concerned about how they are generated. If you need to know, search the Photoshop Help file for "histogram". To get started, open your image in Photoshop and select Mode-Adjust-Levels.

Most of the Levels window is occupied by the histogram Input graph, while at the bottom is the Output bar (adjusting the pointers on this bar will have the effect of squeezing the histogram inwards). The two are used to balance colour and intensity of an image. There are many finer points to learn with Levels, so be patient and experiment.

Levels can work on an entire photo, or, if you select part of the image, it can be applied only to that selection. This is handy if someone's face appears darkened by a shadow, or bleached by a flash.

WASHED-OUT IMAGES

Many over-exposed or washed-out photos can be fixed with a quick adjustment. These images are often characterised by large gaps at either edge of the histogram, or at both ends. To correct this problem, open the Levels window and move the outside pointers towards the centre until they lie just underneath the edge of the histogram. Photoshop will update the photo as you do this, so you can preview the effects of your changes. After adjusting these pointers, the image may become a little darker, or too light, so correct this problem by moving the centre pointer until the image is how you want it.

COLOUR CORRECTION

Images taken under certain lights can pick up too much yellow (light globes) or blue/green light (neon lights). Likewise, as photographs age, they can appear to change colour. Once an image is scanned it may be necessary to adjust the colour channels to restore the image's natural colour balance. This can be done by tweaking the histogram - it may be worth trying the Auto button first, as this can often fix the problem. If it is not successful, examine each channel and see if there are any obvious places to start (to switch between channels, click on the channel drop-down menu). Otherwise, select a channel and start with the centre pointer. Slide it left and then right to see if this improves the colour. Remember that the channels work together to generate the image's colour, so you may need to switch between the channels several times to get the right mix.

SEPIA AND OTHER TONES

You can also give colour or black and white photos a sepia tone using the histogram. If you have a colour image, convert the image to black and white by selecting Image-Mode-Grayscale. Click OK to confirm that you want to discard colour information. Next select Image-Mode-RGB Color (you need to do this with black and white images, too). To bring up the histogram, select Mode-Adjust-Levels. Sepia tones can be introduced by switching to the blue channel and moving the middle pointer of the histogram to the right - but not too far, or the image will turn bright yellow. Add more subtle variations by then switching to the red and green channels and moving their middle pointers a little to the left. Photos can be given other tones, such as magenta or electric blue, by the same method.

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Scott Mendham

PC World
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