Chromatic Aberration

[Haftred]

What is the approximate difference in the points of convergence between red and blue light when using a spherical lens? Is it negligible (i.e. less than a millimeter)? Is it a specific distance or is it a function of a separate variable?

[Reed]

I'm not sure on an exact measurement, but I wouldn't call it negligible. If it were, camera lens manufacturers wouldn't employ the use of aspherical elements and low-dispersion glass.

[brewnog]

Nothing more to add on Reed's comments, but it's definitely not negligible, and it's definitely measurable. The distance would be a function of lens geometry.