Every once in a while, before falling asleep I enter a "mellow" state in which I can conjure mental images dramatically more vividly than normally. I would say that normally it varies between 10-50% of vividness (also depends on the state of mind, the day, whether I'm under the influence of different chemicals), and in this state of mind it's 90%+ vivid.

And I don't confuse this state of mind with lucid dreaming: I had spontaneous lucid dreams a few times. They were not very vivid, just as my normal dreams (I dream in images, and they are maybe around 50% vivid).

So, there is definitely a way to experience variation of vividness of mental imagery, by a single observer.