I tried grinding a custom lens out of acrylic glass (diameter 20mm, focal length 30mm). Not very surprisingly, the result is a quite poor quality lens. I think it mostly suffers from geometry and material defects, while the surface quality is relatively good.
However I'm confused about the difference I observed between two configurations:
I got very poor image quality when I tried to use it as a single-element camera lens:
(Image by DrBob, GFDL, https://commons.wikimedia.org/w/index.php?curid=9690292)
I tried to add an aperture to increase the image quality, but it didn't help much, just made the image darker.
However, I got relatively ok image quality when I placed it in front of my regular camera lens, as a sort-of close up filter (lens 1 in diagram below):
(Image by Tamasflex, CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File%3AClose-up.png)
Why is the image quality better in the second case? Is there a general way to estimate the relative effect on quality of each lens element?