Interseting comments about the variability of our perceptions of tones depending on what surrounds them. Brooks Jensen of LensWork just recently spoke about this on his blog http://www.lenswork.com/lwb.htm And posted a really interesting picture/illustion to demonstrate this: http://web.mit.edu/persci/people/adelson/checkershadow_illusion.html
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re middle grey 50%
2005-11-01 by Scott Jones