Consider two identical pieces of paper.
Scenario 1: On both something is drawn in black ink. If the difference between the areas covered in black ink is sufficiently small, I cannot see the difference between the two drawings.
Scenario 2: Now imagine that the same parts are covered with ink, but their color differs. Again, if the difference is sufficiently small, I cannot see the difference.
Firstly: what causes this? I am aware of the terminology of just-noticeable differences/limina, but do not understand precisely what causes them. Is there some discretization of continuous signals going on that causes `nearby signals' to be treated the same way?
Secondly: one person may have better vision than another, but are there physical constraints that are the same for any two persons and that imply that if the difference in scenario 1 or 2 is sufficiently small, no two persons will be able to see a difference?
Addenda: I phrased the question informally by not defining what a `small difference' is, trusting/hoping that this does not lead to confusion. If desired, such statements can be made precise by adopting appropriate distance functions. In scenario 1, one way to measure distance between two areas covered in ink could be to use the Hausdorff distance; likewise, one way to measure the distance between colors in the second scenario could depend on the difference in responsivity of the three types of cone cells.