Inaccuracies of exposure, variations in illumination set-ups, and also the document scanner's limited ability to cope with the reverse tonalities of a negative image all play their part in the process. Providing there is detail and evidence of shades ranging from dark to lightish tones and that it is fairly sharp, the chances are that a decent image can be created. If you have graphic software such as Photoshop, Picture Publisher or Paintshop etc., this can be achieved easily using the 'Tone Balance', or 'Levels' feature. These show in graphic form the distribution of tones in the photo from black to white in 255 grades. Typical example of under exposed image showing limited tonal range.
Image lacks density owing to under exposure of negative and also limitations of scanner.
Drag the 'shadow' and 'highlight' cursors to the extremities of the curves and click 'ok' to expand the tonal range. The central midband cursor can be moved slightly if required. The result shows the expansion of the tonal range to a more normal distribution of 255 points and provide a more faithful and realistic image. This tonal expansion feature is a very useful and indispensible tool and should be able to correct any shortcomings in the majority of images, but the 'Auto Clip' setting should be avoided as it can exaggerate contrast. The following images are my first attempts and represent an average selection of results obtained using the simple methods described above.