Made the mistake of watching an +Eric Dubay video. I'd heard the convex lens argument before on a +Stefan Molyneux video and was ready to write it off as the guy in that video simply being new to the whole movement.
You know the saying, "You never see a gray one."? It usually refers to the life expectancy or tenure of a certain occupation, especially a very dangerous one. With conspiranoids, especially flat-earthers, a similarly apt statement would be, "You never see a smart one." I bring this up because I'd been giving Dubay the benefit of the doubt that he might be the most intelligent flat-earther, to such an extent that I don't think he genuinely buys into his own bullcrap (i.e. John Norman isn't a misogynist, it's just what sells books) and at worst is simply caught up in his own tabula rasa metaphor.
I was wrong.