Thx for replying and explanation - yes I can see it would work that way it just doesn't sound to be as good as bracketing the exposure and using the higher range of tones available at the brighter end of the scale. I will have to conduct some experiments with both methods (when I get time) see if I can see any difference.

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html