One of the main difficulties of false colour infrared photography, that is, photography where you are recording a mix of both visible and infrared light, is that most lenses do not focus infrared and visible light at the same point. This results in a chromatic aberration effect in images that cannot really be corrected.
In visible light only, lateral CA is relatively easy to correct as the wavelengths of light are recorded in separate red, green, and blue colour channels. But when mixing visible and infrared light together, the infrared does not have its own specific colour channel but is recorded in all 3 colour channels. And so if it does not focus at the same point as the red, green, and blue light, there is no way to adjust it.
Schneider Kreuznach make a line of lenses that are meant to be optimised for working from 400nm-1000nm, the Xenon-Emerald series. These lenses are not cheap and they are quite difficult to find available for purchase, as they are intended more as industrial lenses than for recreational photography.
I recently managed to get hold of the 28mm Xenon-Emerald LD lens. (They make an SD version of the lens designed for close-up work, and an LD version designed for longer focus distances). I already had the 50mm Xenon-Emerald, but wanted a wider focal length. Sadly though, the 28mm does not actually seem to be optimised for Vis + IR work, as you'll see in these tests.
