Focusing in IR
As early as at elementary school, we were taught that each kind of light operates at a different wavelength. The light must pass through the lens, consisting of several lenses, in order to get as far as the sensor of the digital camera. Less expensive types of lenses include those made of plastics. It is generally true that the more expensive lenses the better – i.e. ”clearer“ glass, better surface treatment, etc. If the light of various wavelengths passes through the lenses, it is refracted in a different manner in each lens, resulting in a chromatic aberration. This imperfection is shown itself also when taking pictures in IR. Most of the medium- priced and expensive object lenses are provided with a more or less resistant coating to correct this imperfection. These are usually marked as APO, ED, etc. and are better for taking pictures in IR also because they are more resistant to reflections and other negative phenomena caused by penetration of lateral light into the lenses.
