The “bad eye” verses

I came across this analysis of the “bad eye” verses in Matthew 6:22-23 by John Piper. It makes a lot of sense in the context of Matthew 6 (in concert with Matthew 20:15), but I’d really like to know what you guys who are currently studying Greek make of what Piper says in light of Luke 11:33-36, which is quite similar to the Matthew 6 passage. Thanks!