>  I don't think it will be soon that we require that sort of accuracy for magnification measurement!

Agreed.  I'm uncomfortable just changing the unit test for this to a worse accuracy.

> Do we know that the Lanczos5 is better for shearing accuracy?

I had thought that was part of the justification for preferring them, but maybe not.  Maybe Quintic is always better.

However, I'm currently investigating this.  Including whether going to second order in the flux correction helps.