Predicting Axial Color

If you know your focal length and your dispersion (V number) one can compute the defocus due to color, otherwise known as axial color


This can be shown in a refractive lens as:
The rays are colored assuming visible light, but this works for IR, UV etc.....

If this is a diffractive element, the color is reversed due to the sign change in the V number

Reference
Field Guide to Geometric Optics, By J. Greivenkamp