Everything you say makes perfect sense to me.
Thanks for the idea about testing, I really should compare my 4X objectives as you did. I must have 6 or more, and I don't remember how they perform.
I am also curious to see how the PlanAPO 4X versions compare. I have old and new finite and infinity types. The newer finite version of the 4X PlanAPO seems to have an even smaller image circle than the newest infinity version!
testing and comparing lenses/objectives is one of my favorite hobbies
Truth be told the Amscope does not come off badly at all in my comparison, as far as I remember, center resoluton-wise it was outperformed by Nikon CF N 4/0.13, Nikon CF N PlanApo 4/0.20, Mitu 5/0.14 and "tied" with some good bellows/enlarging lenses. Again, not surprising given the higher NA of some of its rivals. Coverage-wise it clearly outperformed the Nikons (no surprise here either given the poor image circle of the CF Ns). Just for the record, at 5X it was slightly outperformed by my old and beloved Nikon U10/0.22 stopped down at around f/3.5, coverage was a tad worse though.
Yep, the newer finite PlanApo 4/0.20 (160/0.17) has the same (or worse) poor coverage as the CF N version
Some years ago I used a Nikon CF N PlanApo 4 with a diaphragm I attached to it, the result was not bad at all, you obviously lose some resolution, but coverage improves. I have to repeat that experiment