Special relativity involves comparing what different observers see. But we need to be careful about what these words mean.

A reference frame is a way of labeling each event with its location in space and the time at which it occurs. Making a measurement corresponds to recording these labels for a particular event. When we say that an observer “sees” something, what we really mean is that a particular event is recorded in the reference frame associated with the observer. This has nothing to do with actually seeing anything, a much more complicated process which would involve keeping track of the light reflected into the observer's eyes! Rather, an “observer” is really an entire army of observers, who record any interesting events; an “observation” consists of reconstructing from their journals what took place.