Say that you enjoy looking at two beautiful paintings (A and B). But you really can't tell which one you like more, and you want to (quantitatively).
So, you compare them both to your favorite painting (C), which you know you really like to look at (adjusting for novelty).
In order to do this, you vary the size of your favorite painting C on a computer screen (or its distance from you in real life), and find the point at which C is so much smaller that you'd rather look at A. You do the same for painting B.
You then attempt to quantify how much you prefer A to B by asking: for which painting (A or B) was C larger when you first preferred to look at that painting rather than C? This is the one that you prefer more, and the distance between the C "breakpoints" gives some measure of your relative preference.
Of course this difference will be at best an interval measurement, not a ratio one, but that's usually better than no measurement.
Here is my question: what is this procedure (or some variant of it) called? This seems like a basic enough test that it must already be in use, and I am curious to hear more about it.
Related question: How is the size of a video related to its perceived quality?