[lcrs]

I've used a triangular ("conical"?) kernel when in a hurry and it looked reasonable. Vastly better than a box blur, at least. They're actually not separable but one can pretend and just do horizontal and vertical passes without much damage.

Amusingly Autodesk ship a GLSL shader with Flame which uses a triangular kernel under the hood but labels the button in the interface "Gaussian", the cheeky swines...

The nicest option to imitate lens blur is a circular kernel but that's quite aggressively non-separable. The other thing that really helps is blurring linear-light values instead of the usual gamma-encoded images. The blurs in Windows 7's Aero themes very clearly don't do this, to my eye - dark lines spread out too much and look generally muddy.

VFX guy who loves to blur things here :)