Clive R. Haynes FRPS


'Levels' offer a greater a greater degree of control and sophistication than the rather crude 'Brightness & Contrast' adjustment (which affects the whole image area equally).
They are almost as good as 'Curves' for image correction but lack the versatility and degree of fine control that 'Curves' offer.

'Levels' have five points of control (three 'input' and two 'output'), with the added sophistication that we can set both the 'black point' and the 'white point' and have control over the mid-grey setting.
Let's begin by looking at the 'Levels' dialogue box and see what the 'controls' offer

To explore the use of 'Levels' we'll use an image that also appears in the section about 'Curves'. In this way a comparison may be made.

In this version of the lh image we can see that by using the lower ('Output') gradient as a guide, the position of the darkest and lightest tones in the image can be identified - both fall short of the full range
(The red arrows indicate this)

Above: The opening state of the 'Levels' dialogue box for our demonstration image.

Note the flat lines at each end of the histogram. These represent the darkest tones (lh) and the lightest tones (rh) within the image. The darkest tone falls short of absolute black and the lightest tone falls short of absolute white.
What this means is that the image will lack 'punch' or contrast as the darkest tone will be almost black and the lightest tone a very light grey.

The triangular sliders can be moved to set the 'black point' and the 'white point'. Doing this will improve image contrast.
The mid-point for the image 'exposure' can be set with the centre triangular slider without adversely affecting the black or white points of the image.

'Levels' therefore introduce a subtle form of control.

What happens when we move the sliders?

Above: Levels adjustment
Above: The Levels dialogue box - after setting
Above: What we have done is to move the lh 'slider' so as to 'fix' the black point to the lh 'tail' of the darkest piece of useful information in the histogram. We have also moved the rh 'slider' so as to 'fix' the white point to the rh 'tail' of the lightest piece of useful information in the histogram.
By reducing the distance between the black and white points, the contrast has increased.
The image now has a full black and an absolute peak white.
The mid-grey point has remained in the original position.
To discover more about 'Levels' - click the link below

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