Brightness & Contrast

The most basic tool that you can use to adjust the tonal quality of an image simply allows you to adjust the brightness and contrast. This can allow you to correct for minor exposure errors in the picture.

ConferenceConsider this image. The problem here is that the shadows in the hall are quite dark and the screens are bright because of the intensity of the projections. As a result it is hard to get a single exposure that captures all of the detail in the scene.

We can use a basic brightness and contrast adjustment to help correct for this. In Photoshop the required tool can be found on the Image>Adjustments menu. In the GIMP this tool can be found on the Tools>Colour Tools menu and also directly on the Colours menu.

Brightness dialogBy adjusting the brightness of the image upwards by 50 we can increase the detail in the shadowed area significantly. However, increasing the brightness on its own can sometimes create a washed out appearance in the image, so it is usually a good idea to increase the contrast a little at the same time. There are no fixed rules for decided how much each parameter needs to be adjusted by, using the Preview option allows you to see the effect directly in the image as you try different settings.

Unmasked brightnessThe result of this adjustment can be seen on the right. There is now much more detail visible in the areas that were previously shadow, which improves the image. However, as a result, the screen is now overly bright and has lost some of the depth of colour that it has in the unadjusted image.

If there was a way we could adjust the brightness and contrast of just part of the image, leaving the screen unaltered we could achieve a much better final result.

Luckily there is such a technique. By using something called an adjustment layer along with a mask it is possible to select exactly which portions of the image are altered.

Masked adjustmentI will discuss the details of these techniques later, but the final image shown here demonstrates the result. The detail in the shadows has been improved whilst the colour and exposure of the screen has been retained.

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