Brightness

The photograph below includes a lot of sky and therefore the Buddhist statue (at Repulse Bay, Hong Kong) has come out rather dark. You might be tempted to use the Brightness/Contrast panel to improve the picture. Don't! When you increase the brightness in this way, light is added to every part of the picture, so you will find that parts of the picture that should be pure black have become grey - pure black is no longer available. Similarly, using the Brightness control to make an overly-bright picture darker would also be a mistake, for pure white would no longer be present anywhere in your picture. In other words,the Brightness control is always to be avoided!

There are two other, better and more sophisticated alternatives for changing brightness. The first is to apply a 'curve' to the picture and the second, as we saw in the previous lesson, is to adjust the 'Levels'. Let's try thecurves control first.

Choose Colors> Curves and make sure that the Preview option is turned on. Notice that is already a point at each end of the 'curve' - we will usually not touch these two points. Use your mouse to drag the center of the diagonal line into a new position, thus adding a third point, and note that it is possible to brighten or darken the image without losing the pure blacks and whites in the image. If you want to play, you can try pulling more than one point on the line (some very weird effects are possible), but if all you want is a simple brightness correction then a single added point should be enough (you can't get rid of points that you have added except by resetting everything so think twice before adding additional points).

For this particular photo, it would be nice to lighten the shadows while leaving the sky more or less unchanged, and this can be achieved by adding two points to the curve like this:

Notice how the lower part of the line (representing the darker parts of the photo) has been lifted above the original 45 degree line, while the top part of the line has been left in its original position.

If you're like me, you'll probably find the colors of the statue are rather pale. To make them more intense or 'increase their saturation', choose Colors>Hue-Saturation and then turn up the saturation using the corresponding slider. As usual, make sure the preview option is turned on. Don't be tempted to adjust the brightness here, for the reasons we have already discussed. This photo doesn't need a 'hue' (tint) adjustment, but try it anyway to see what it does.

As you discovered in the previous lesson, the Levels control can also be used to adjust brightness, mainly by moving the middle triangle under the graph, but we could not have used the Levels control to get the result we achieved here i.e. brighten the statue without brightening the sky. Thus it would seem that the Curves control is the best control for adjusting brightness, but don't forget that the levels control has the advantage that it allows us to make sure that our image is using the full range of brightness possible, all the way from pure black to pure white.

Decreasing contrast

As promised in the previous lesson, I will now suggest a good method for reducing the contrast when necessary. As stated the previous lesson, the use of the contrast control is not recommended for it will cause your picture to lose any pure white or black that it contains, making the picture look flat and lifeless. The better method is to use the curves control introduced above.

Copy this picture (a tiny Japanese drinking water fountain) to your folder.

Although the overall brightness is OK, the high contrast makes the picture rather harsh and makes it difficult to see detail in the leaves to the left of the fountain. Open the picture in the GIMP and display the curves control. To reduce contrast without losing whites or blacks, make a four point curve like this:

The central part of the curve is less steep than before, and it is this that corresponds to reduced contrast. This picture doesn't need much adjustment so don't overdo the correction. If you care to check with the level control, you will find that the picture has not lost its pure whites and blacks. Save your work.

It should be clear from this lesson that these are powerful tools that are sometimes hard too use. When you reach the Adjustment Layer lesson I will show you how you can use the curves and levels controls in such a way that you can easily return to them later to make adjustments.