I observe mostly from my mid Tokyo location where light pollution is literally off the charts. (I recently bought myself a Sky Quality Meter and much to my dismay its readings have been consistently around 16 or thereabout, which seems much worse than is typical for Bortle-9 skies.)
I've leaned that an effective workaround for city light pollution is to increase the power and reduce the exit pupil. With the rainy season finally starting to come to an end at long last, I just had a chance to conduct a simple experiment to see how brightness changes with exit pupil size while keeping the mag constant. In this experiment, I used my Nikon 18x70 as the test instrument and put on step-down rings of varying sizes to change the exit pupil. (There's a key reason why I used my Nikon 18x70 as the test instrument: I've been seriously thinking about buying a Canon 18x50 IS.)
Step-down ring size Exit pupil size
What I found was that at exit pupil size of 2.9mm I started to notice that the view was really dim and that I was having difficulties spotting some of the stars. At 2.7mm, it was really too dim for my taste. (It occurred to me then that 2.7mm is about the exit pupil size of the Canon 18x50 IS. This experiment caused me to re-think my plan to get a Canon 18x50 IS. I'll repeat the experiment another night, and if I continue to get the same results then I'd have no choice but to change my plan and get a Canon 15x50 IS instead.)
While it should be no surprise that reducing the exit pupil size would lead to dimmer views of the sky, what's surprising (to me at least) is that I have had no issues at all with the views through my Kowa Highlander Prominar at 32x (exit pupil = 2.6mm) and 50x (exit pupil = 1.6mm). Never once while looking through my Kowa have I had the thought that the view is too dim for me to make out the stars. In fact, it has been the exact opposite: I can see a lot more stars at 50x compared to 32x.
So what is going on here? Why would I have no trouble whatsoever with an exit pupil of just 1.6mm in my 50x82 Kowa but find the views through my simulated Nikon 18x48mm too dim? Is it true that small exit pupil sizes need to be compensated with much larger objective sizes to prevent the views from being too dim?
Thank you in advance for your explanations.