> This is a 0.03% error. We have to decide if that's tolerable (in which case I'll change the unit test and push the updated code) or not.

I would guess that if for shear we're aiming for ~2e-4 additive-type biases then 3e-4 fractional size errors is kind of at the limit of what we would like to tolerate for a magnification, but maybe for a first go it's tolerable.  I don't think it will be soon that we require that sort of accuracy for magnification measurement!

I'm trying to wrap my head around why this is happening though... This whole non-flux conserving stuff does make the Lanczos quite non intuitive.  I do have one question:

>However, I suspect we care more about fidelity of shearing accuracy than flux conservation, so I think we probably want to go back to having Lanczos5 as the default and just point out the issues with the flux.

Do we know that the Lanczos5 is better for shearing accuracy?  They seem to be performing very comparably at least in Melanie's tests on #389 ...