If youve never encountered it before the levels control will amaze you with its ability to make a flat lifeless shot suddenly spring to life Not only is it one of the most effective tools to use its also one of the quickest

It's often the case that a shot you thought was going to come out looking great will end up with a net-curtain effect The reason for this is a lack of contrast Fortunately, it's easy to correct using the levels control.

Any half-decent photo-editing package includes a levels tool. Here, we're using the free, open-source package the Gimp. First of all, select the levels tools from Tools | Colour Tools | Levels.

The histogram that appears represents the distribution of pixel intensity in your image. A low-contrast shot will have most of the pixels clumped together. We can use the levels tool to remap them.

Just drag the left-hand slider beneath the graph inwards until you reach the point on the histogram at which the concentration of pixels increases. This remaps what were dark greys down to black, immediately boosting contrast

Now try the same with the right-hand slider, although you may find that when you start pulling it to the left, overall image brightness increases too much. If thafs the case, slide it back until the unwanted effects disappear.

That's all there is to it The levels tool works wonders for some images, but will improve almost every shot you use it on, which is why you should apply it to every photo you want to print or display on the web.

It's a cbched shot that every visitor to Australia takes...

...but you can make it stand out from the crowd.


It's a cbched shot that every visitor to Australia takes...

histograms and tonal range it's easy tO Switch off aS

soon as people start mentioning concepts such as tone, but expanding an image's tone to cover the full dynamic range available Is the most Important overall adjustment you can make. The concept of tonal range is, in fact, simple: it means the darkest pixel levels of an image in comparison to the brightest.

You can assess the tonal range of an image by using the histogram display. Like the levels and curves controls, any decent photo-editing package will let you see the histogram (see picture below). It's a bar graph showing the number of pixels at each intensity, from totally black to maximum white. A standard JPEG is what's known as an 8-bit image; this means the lowest pixel intensity value is 0, while the highest is 255 (representing bright white). 128 is mid-grey.

Typically, the intensity of a shot as it comes out of the camera will be clumped in a central part of the range, assuming it's properly exposed (see p70). If it isn't, the histogram will have most of the pixels clumped to the right for an overexposed (overly bright) picture, and to the left for an underexposed (overly dark) one. In a typical image, most of the pixels may be in the range 70-180, for instance. That means the image isn't taking advantage of the full

The histogram shows the tonal range of a photo.

...but you can make it stand out from the crowd.

range of pixel values and, when it's rendered onscreen or printed, the highlights and white areas will look greyish.

In short, the image will lack contrast. So we need to expand the tonal range, so that (in our example) pixels with a value of 70 in the original image are brought down to 0, and pixels with a value of 180 are increased to 255 (the maximum value in a standard 8-bit image such as JPEG). Then the pixels In between are redistributed proportionately. The upshot isn't only an increase in richness and impact: detail that may not have been obvious is also brought out, since pixels that were clumped together are separated. The result is a much-improved image, which can be achieved in just a few seconds.

Since the tonal range and its adjustment are so important, there are two standard tools to adjust it that are available in any decent photo-editing application: this includes the free open-source Gimp, Paint Shop Pro Photo and Photoshop.The simplest and quickest is called the levels control. Its primary purpose is expanding the tonal range evenly across the full width available. At its simplest, it requires that you adjust only two sliders. See the walkthrough on pi 15 for how to use it. You'll note that we've only talked about the overall intensity of pixels here: in fact, an 8-bit colour image is made up of three 8-bit values between 0 and 255 to represent red, green and blue components of the picture. You can alter the levels individually in most software, but usually you'll end up with a hideous mess, since the colour balance will immediately be thrown completely out of whack.

curves The second and most powerful tonal control is called the curves tool. This can either replace the levels control or be used in conjunction with it. The advantage over the levels tool is that, as well as expanding the dynamic range of an image, it can manipulate the tonal response across the whole range. While levels simply expand the response evenly across the image, with curves you can control the response so that, for instance, shadow detail is reduced to black, while highlights are increased, but midrange tones remain unaffected. In fact, this is the classic use of the curves control, known as the S-curve. It boosts contrast and increases the impact and mood of a shot. Curves is often the first adjustment any professional or serious amateur photographer will make to an image. See the walkthrough opposite for more.

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