At the risk of drifting off subject, the IR spectrum is about 10% off from the center of visible light spectrum. Red is close to IR. Green is about the middle.
The Coastal Optics 60mm f/4 lens is one of the best lenses made. It is designed to eliminate focus shift across different subsets of the spectrum - including UV and IR. It is a manual focus lens costing nearly $5000 - but it is optically fantastic.