Hey all, In chapter 6 section 3 of Modern Optical Engineering, 4th edition, by Warren J. Smith, it claims you can calculate all the 3rd order aberrations by considering two paraxial rays. I'm trying to convince myself of this and not having much success. Unfortunately, the article that this book gets this info from is subscription-walled to me (Article: https://doi.org/10.1364/JOSA.41.000630). My question is, how would you explain that you can get 3rd order aberrations from two paraxial rays, even though the whole problem with aberrations is that they are missing from the paraxial approximation? Is it just some algebraic coincidence, or is there an intuitive reason?