In applications where absolute judgement is required, the best we can do with a single graphical attribute is between 4 and 7 values. To get a larger range of recognizable levels, we must repose the problem in multiple dimensions, do a sequence of simple decisions, or perform some type of chunking.
Alternatively, we could redefine the problem in a way that relative rather than absolute judgement could be used to focus attention, with a second, more quantitatively accurate, stage following the initial focus of attention.