The Levels dialogue allows you to adjust brightness and contrast levels, and more. Open it any of these ways:
- Image window > Colour Menu
- Image window > Tools Menu > Colour sub-menu
- GIMP window > Levels Icon (Install it!)
Before using the Levels dialogue it is worth understanding why you should use it in preference to the sliders on the Brightness-Contrast dialogue.
This graph plots every point of the picture in terms of its brightness on a scale of 0-255, with vertical guides marked at intervals of 50 units. The number of black points (0) is shown on the left and white (255) on the right, with various levels of grey in between. Where many pixels are of the same brightness the plot stacks them to form characteristic "mountains".
Some cameras fail to record the extremes of black and white on some images. These photographs will appear bland and lacking in contrast. The illustration on the right shows a typical example.
To maximise contrast without losing any picture detail the end the light and dark points need to be adjusted.
To increase brightness at the brighter end of the scale, drag the white triangular marker (under the right end of the Input Levels graph) towards the left. To increase the intensity of of the shadows and other dark areas, drag the black triangular marker (under the left end of the graph) towards the left.
In most cases, the best effect will be when the pointers are under the start of where the graph begins to rise from the base line. In this example, the dark point should be set at around 35 and the light point at about 225.An alternative to dragging on the pointers is to adjust the numbers in the fields under the light and dark point.
Clicking the OK button will accept the changes and close the dialogue. Some photographs will benefit from some brightness correction as well. Make those adjustments before clicking the OK button.
While adjusting the light and dark point markers to improve contrast, you will have noticed that the central grey marker has moved in sympathy, and that its value has remained fixed at 1.00.
Having set the contrast then drag the central marker left or right. Dragging to the right expands the distance to the dark point and so makes the image darker. Similarly, dragging to the left makes the image brighter. However, neither of these moves off the ends of the scale, they merely increase or decrease the range, hence no image detail is lost.
In a case such as the example illustrated, the peak on the lighter side of the grey mid-point is so pronounced that it indicates that the excess brightness could probably benefit from being reduced (compressed), Hence the grey marker should be dragged to the right reducing the value below 1.00.
When satisfied with the result click the OK button.
The Levels Dialogue will help cure most light level and tonal problems, not just brightness and contrast. For photographers, the following two controls are the most useful:
So far, the drop-down Channel list in the top left corner of the dialogue has been left at its default "Value" setting. Changing this to one of the individual channels for red, blue and green. Use this to correct white balance and other colour casts in your photograph, by making adjustments of the brightness and contrast of one or more channels independently.
NOTE: Often the colour effect desired is not best achieved by altering the colour in question, but by adjusting the other two colours in the opposite direction.
In the image above the apparent boost to the green channel was achieved by adjusting the gamma setting for the blue channel to 0.60 and the red to 0.90.
If adjusting the Input Levels is a bit like making adjustments to a TV projector, then adjusting the Output levels is a bit like putting on different different grades of sunglasses or coloured spectacles to overcome problems with the Input Levels. The Output Levels control allows you to remove both the darkest and brightest light levels of the entire spectrum or that of individual colour channels.
In the example above, the sky was massively over exposed and should have been a clear blue. Starting with the left hand image, the sky was first selected in order that the remainder of the image would be unaffected. Then the light point output level of the red channel was reduced to 180, the green to 217 and the blue to 236 to achieve a blue sky. (These values were chosen as they were found at the top of a properly exposed photograph taken a few moments later.)
1. The technique described here is not the preferred method for recovering the colour of an over exposed sky. It is used simply as a demonstration of the use of output levels. A real blue sky is not a plain colour, but gradually gets lighter towards the horizon. To produce that effect it is probably easiest to introduce a layer with a graduated tint. However, in this picture the trees are sufficiently tall to obscure the true horizon and the casual observer might not detect the fault.
2. To further enhance this image, work also needs to be done to reduce the over exposure of the fringes of the tree tops by selecting the affected areas and reducing the output levels.)