Why is infrared light's focus point different from that of visible light?

Question

When reading about infrared photography, it's often mentioned that IR's focus point is slightly different from visible light. Why is the focus point of infrared light point different from the focus point of visible light?

Answer 1

It is for the same reason that chromatic aberration occurs at all: different wavelengths of light will bend at slightly different angles when passing through the same refractive medium such as a lens element. Chromatic aberration in most well designed photographic lenses will be less severe because the lens has been designed to correct for it at the various wavelengths of visible light and because the difference in wavelengths between one end of the visible spectrum and the other are not as significant as the difference in wavelengths at the center of the infrared spectrum and the visible light spectrum. There are specialty lenses designed particularly for the longer wavelengths of infrared light (also lenses for the shorter wavelengths of UV light) but they are intended primarily for other applications than the type of photography covered within the scope of this site. They are also prohibitively expensive for most photographers, either hobbyists or professionals.

Infrared light requires a different focus setting in a lens because the wavelengths of infrared light are significantly different enough that the refractive properties of the lens will bend it at different angles than what they bend the various wavelengths of visible light.

Answer 2

The ideal lens would cause light beams of every color to come to a focus at the same distance from the lens. That would be the focal length of the lens when the lens is imaging at infinity (∞ as far as the eye can see. When we image objects that are closer than infinity, they come to a focus further away from the lens. That is why we must cause the camera lens to move forward, away from film or digital sensor when focusing on nearby objects. This is due the fact that lenses have limited power to refract light (cause to bend inward). In other words, objects closer than infinity require a longer distance to focus. We are taking back-focus (distance lens to focused projected image).

The fact that a lens has limited ability to refract light is even more complex when it comes to colors. As a matter of fact, each color comes to a focus at a different distance from the lens. Blue images closer to the lens than red, and green, yellow, orange, etc. occupy intermediate positions. Now the further away from the lens a color focuses the larger that color’s image will be. We are taking chromatic aberration. Because the red image is slightly larger and blue image is the smallest, we see color fringing around objects. In other words we are unable to focus our camera on all colors simultaneously.

Now a convex lens has the opposite chromatic aberration than a concave lens. This fact allows lens makers to construct the camera lens using a combination of positive and negative lens elements. Also different hardness of glass (density) are used to make up the array of lens elements in the lens barrel. Crafty use of different glass and lens shapes mitigate but never eliminate chromatic aberration. Infrared focuses further from the lens then the other colors and ultraviolet focuses much closer to the lens than the colors. Special lenses optimized for UV and IR are possible but these are reserved for scientific applications. Most camera lenses are highly corrected for most all the aberrations, there are seven and you can look them up. 1. Spherical, 2, Coma, 3. Astigmatism, 4. Curvature of field, 5. Distortion, 6. Longitudinal chromatic 7. Transverse chromatic.

Again all aberrations can be mitigated but none can be eliminated.