The visible spectrum of light consists of a range of wavelengths from 400 nanometers (nm) to 700nm. Below 400nm is UV radiation and X-rays and above 700nm is infrared (all capable of being recorded photographically). When the visible spectrum is viewed simultaneously we see ‘white light’. This broad spectrum of colors creating white light can be divided into the three primary colors: red, green and blue. The precise mixture of primary colors in white light may vary from different sources. The light is described as cool when predominantly blue, and warm when predominantly red. Human vision adapts to different mixes of white light and will not pick up the fact a light source may be cool or warm unless compared directly with another in the same location.
The light from tungsten bulbs and firelight consists predominantly of light towards the red end of the spectrum. The light from tungsten lamps is also predominantly light towards the red end of the spectrum. The light from flash consists predominantly of light towards the blue end of the spectrum. Daylight is a mixture of cool skylight and warm sunlight. Image sensors balanced to ‘Daylight’ will give fairly neutral tones with noon summer sunlight. When the direct sunlight is obscured or diffused, however, the skylight can dominate and the tones record with a blue cast. As the sun gets lower in the sky the light gets progressively warmer and the tones will record with a yellow or orange cast. The color of light is measured by color temperature, usually described in terms of degrees Kelvin (K). This scale refers to a color’s visual appearance (red - warm, blue - cold).
The advantage of fast opened lenses is that one can shoot in the existing ambient light without a flash or any other light source. The existing tungsten light results in warm feel to the photograph.