> Grab a piece of 100% white cardboard and another piece of 90% gray
> cardboard.
>
> Now dim the light to half brightness. Obviously, your white cardboard
> will now reflect only 50% of full-brightness white into your eye, and
> your gray cardboard will reflect only 45% of full-brightness white.
>
> So the absolute difference between the two pieces of cardboard has
> diminished from 10% full brightness to 5% full brightness. But our eyes
> are designed and trained to look not for the absolute light intensity,
> but for /relative/ difference, so that we can identify the "pigment" of
> objects irrespective of lighting conditions. The /apparent/ brightness
> difference between 45% and 50% is therefore roughly the same as between
> 90% and 100%.
>
> As a result, a truly linear gradient doesn't /appear/ linear to us: The
> "distance" between 10% and 20%, for instance, is percieved as roughly
> equal to that between 50% and 100%. Go figure.
>
What you are missing here is that this ability for the brain to retain a
constant relation between widely varying changes in size, shape, colors or
perspective is not something you need to correct in the picture. The brain does
it. It works in a photograph as well, which is just a neutral recording device.
The Greeks made this error in their temples, for example. They thought by
flaring out the upper proportions in a trapezoid they were "correcting" the
dimension to make them look right.

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