|Auto White Balance|
Alan's Tip of The Day-
**When color film came on the market in 1935, it was necessary to market several versions of the same film. One version for use outdoor was called "Daylight". Two versions for use under artificial light one for work in the movie industry and one for work by professional photographers using "hot" lights.
If Daylight film was used indoors in a studio lit by tungsten lamps, the result was pictures that were red-amber. If the indoor film was used outdoor the pictures turned out too blue.
We are talking about severe off-color results. Photographers (me too) carried a camera bag filled with color correction filters. There were two sets one set was amber, they adjusted the color of tungsten lamps so the daylight film would work indoors. The other set was blue, used to adjusts films designed to operate under tungsten lamps work outdoor.
An ordinary light bulb uses a metal filament that heats up when electricity is applied. Tungsten is chosen because of all the metals it has the highest melting point. So when we talk about studio lighting we must classify as to what type of lamps are being used and we must guard against mixing different types.
Type of possible lamps:
Ordinary household light bulbs
Photo lamps designed for studio photography "hot lamps".
Photo lamps designed for use in the motion picture studio "hot lamps"
Florescent lamps (many types) Daylight - Cool white - Warm white - etc.
It became necessary to classify all these lamps in some way.
Work had been done a long time ago that correlated the temperature of glowing hot metal to a color chart. Now the confusing part was which temperature scale should be used? By that time, Europe was using the Celsius scale. This sets water freezing at zero (0) and water boiling at 100. There are 100 steps (degrees) between water freezing and water boiling. The other scale, popular in England and America and still used in America only is the Fahrenheit scale. This scale sets water freezing at 32 and water boiling at 212 with 180 steps (degrees) between.
Scientist favor, as does all of the world, the Celsius scale. Both the Celsius and the Fahrenheit scale have announces. The key dislike by scientists is cold weather can be below zero and thus must be written with a negative number like "boy its cold today -15". Scientists don't like the possibility of notes being misunderstood, was it +15 or -15?. Scientists had concluded that -273C was the coldest possible temperature. They called this Absolute Zero. They said why not start the temperature scale at -273 and call this zero and use the Celsius scale. This temperature scale was called the Absolute scale and latter changed to Kelvin Scale to honor Lord Kelvin a celebrated English scientist who did lots of work on temperate. The Kelvin scale is used today in the scientific community. Because hot metal glows when heated, the metallurgy scientist use it a lot. Now light bulbs use metal filaments so this industry took to the Kelvin scale to classify artificial light sources. Photography, using lots of artificial light adopted this method.
Film and digital sensors are more sensitive to blue light than any other color. Blue light has the shortest wavelength and it is the most energetic. It is blue and ultraviolet that gives you a suntan.
The chip in the digital camera records mainly blue light. To correct this flaw the chip is covered with filters using a scheme invented by Dr. Bauer of Kodak. The scheme uses an uneven number of red - green - blue filters in an array on top of the chip. Nevertheless, when we take pictures the odds are the color will be wrong. The digital camera has an electronic method that checks the color of the light before the picture is taken. In the camera are red - green -blue amplifies, like volume controls. This software attempts to adjust the amplifiers to produce images of white objects that look white in the final picture. This mechanism is called White Balance. It is imperfect. This lesson deals with manual setting of the White Balance.
The human eye/brain also has the three amplifier systems, actually two of them, one for the left eye and one for the right eye.
Procure some strong colored cellophane. Candy wrappers or red/blue 3D glasses or camera filters etc. These must be strong in color. Find a red candy wrapper and place it over one eye. You will see the world go red. Keep both eyes open and look about. Remember the red cellophane is over one eye only. Keep this red filter on the eye for 3 or 4 minutes. Now remove and look about. Quickly cover one eye with a card or your hand. Then switch the cover. In other words, look about using just your right eye and then the left. Switch back and forth. If preformed correctly you will see that the red filtered eye has changed in its color sensitivity. The eye/brain seeing all this red decided that the red filtered eye was seeing abnormally and the eye/brain turned down the red amplifier. The blue and green amplifiers working OK did not change. Now you see with that eye, too blue and too green in other words an absence of red we call cyan. This condition will last only a few minutes. If you choose green cellophane the results will be magenta (red-blue). If you choose blue cellophane the results will be yellow.
This adjustment of the red - green -blue amplifiers of the eye/brain goes on all the time. You are unaware because it happens to both eyes. This experiment caused only one eye to change thus you became aware of this natural phenomenon. Let me add that the eye/brain secrets a sensitizing agent under dark conditions that elevates the eye's ISO 10,000 fold. The chemical is rhodopsin (visible purple). Sailors and airmen sit in red light to force the generation of rhodospin as this improves night vision. Ships and space ships go "condition red", not because of a threat but to better adapt the men to go on deck and see better. Pirates wore eye-patches not because they lost an eye but because they dark-adapted one eye for watch at night.*******My deepest gratitude goes out to Alan Marcus for helping me to understand all these amazing things that my camera is capable of. I am so lucky to have such a great teacher, and happy to be able to share his knowledge with you as well. **********