According to constructive empiricism, "X is observable if there are circumstances which are such that, if X is present to us under those circumstances, then we observe it" (van Fraassen 1980, 16). For the constructive empiricist, observability is relativized to "us."
But who should count as "us"? People's sensory qualities are not uniform. Rather, they vary in degrees. Some people can see better than others. Some people's hearing is better than others.
Should observability be relativized to the best observer, the average observer, or the worse observer?