Crimson Longinus
Legend
It is determined by DM for the purposes of each situation and that situation may vary. Doing it otherwise leads to nonsense. standing behind bushes might provide obscurement to viewers from on direction, but not from another. Similarly some things might provide obscurement when seen from ground level, but not from the air. And of course obscurement is subjective, or how do you think darkvision works?Wrong answer! The obscurement level of a given area is not subjective. It is determined by the DM.
In this case it would be the combined minuscule amount of light coming from three differnt sources, two torches and the moon.Since you, as DM, have established that the area between the torches is lightly obscured, what is its light source, given that the illumination provided by the torches does not reach it and the moonlight is not sufficient?
But you're looking this whole thing from the wrong perspective. I'd describe what the characters see, and then assign obscurement level that seems to make sense based on that.