2Tricky:
The technique used (so far) places the light half on one colour and half on the other colour, which provides the average greyscale as output. To move from fully on one colour to fully on the other the image needs to travel around 1.5mm which provides less and less resolution the lower the relative greyscale values get.
This is the first time I've had any idea what you're using the A/D input for. Is it correct that you're using the dark/light contrast to find the position of a boundary between dark and light parts of an image?
How accurately can you position the sensor? Is it feasible to scan the sensor across the boundary? If so, I suspect you could get a much more precise position for the boundary by interpolating the position of the '50%' brightness point from a sequence of brightness readings in different positions rather than needing a single brightness reading to give you a hugely precise measurement of the proportion of dark/light areas within the image.