How light waves focus at different points
When taking an infrared photograph, something to consider is that infrared light focuses at a different point than visible light. When light is shone through any piece of glass, depending on its wavelength it will bend in a different direction. Red wavelengths bend the least, as they are longest, whereas violet light waves are shortest and bend the most. This is how rainbow spectrum are generated using prisms – the glass of the prism bends each different color of light in a different direction, effectively separating them. Not only does the focus depend on the light’s wavelength, but it also depends on the lens’s focal length. If a lens has a fixed focal length, then the infrared focus shift will be constant. However, if the lens zooms between two focal lengths, the infrared focus shift will vary, making it hard for users to focus properly. Because infrared wavelengths focus at a different point than visible light, many lenses include marks designed to help the user achieve perfect focus.