Difference between permittivities eps-opt and eps-inf?

Very often in materials physics we are interested in the relative permittivity at optical frequencies, which is usually denoted by $$\varepsilon_\text{opt}$$ or $$\varepsilon_\infty$$.

But I'm confused because $$\varepsilon\left(\omega=\infty \right)=1$$, since at high enough frequencies the medium does not have time to respond to the field and alter the allowed flux per unit charge.

So to me I feel like $$\varepsilon_\text{opt}$$ is very clear but that $$\varepsilon_\infty$$ is a stupid convention to denote optical permittivity because it should be =1 always...

But perhaps there is something I'm not understanding? Am I missing something or is it just a convention thing?

• And at x-ray frequencies, the refractive index is even slightly smaller than unity. – Pieter Jul 12 '19 at 20:26