ON THIS PAGE:

Colour temperature
Light source table
The importance of white balancing
When to do one
White balance settings
Manual WB
Problems white balancing?

PART 4: WHITE BALANCE

by Christina Fox

Lightbulbs, flouecsent tubes and the sun - all of us see these sources of light as white light . But our eyes are deceiving us, these sources of light vary considerably in their colour. Our brains are very good at performing a white balance without us really noticing. However the camera has to be told what type of light it is working in - so that it can correctly reproduce what our eyes see.

To understand the need to white balance you first have to understand colour temperature...

COLOUR TEMERATURE

If I put a poker in a fire it will start to glow "red hot" - if I put it in a furnace and really heat it up it will look as if it is "white hot". As the temperature of the poker rises the colour of light it emits changes. Red hot is pretty hot while white hot is very hot. There is a definite connection between the temperature of the metal and the colour of light it emits. This "colour temperature" connection is a way of scientifically quantifying the colour of the light source.

Ok now think about a light bulb - inside is a filament (made of tungsten metal). When you switch on the electricity, a large current flows through a small filament, it gets hot and starts to glow. (ie It behaves like our poker). The colour temperature of tunsten light usually lies at around 3,200 Kelvin. It is actually a reddish orange light.

The sun is another main source of light. As you'd expect it is hotter than a light bulb! The temperature on the surfaceof the sun is 5800°K, while inside and especially at its core, it reaches millions of degrees. However, the light that it produces must firstly pass through its own atmosphere and then through the atmosphere of the earth before it reaches earth's surface.

So, daylight consists of a mixture of the light that comes directly from the Sun with the indirect light that comes from diffusion and reflection, caused by earth's atmosphere and clouds. This makes colour temperature on the ground pretty variable from as low as 2,000 Kelvin up to 20,000 Kelvin. The table below shows the range of temperature possible for daylight and the conditions that effect it (along with some other light sources).

WHY IS WHITE BALANCING IMPORTANT?

Well, we have to tell the camera what colour of light it is working in so that the picture it records looks something similar to what our eyes see. To do this we need to do a white balance.

When the camera does a white balance - it analyses the spectrum of colours hitting a white piece of paper. It juggles these until the white looks white.

In areas with a single source of light this is pretty simple. When we have mixed sources of light we can have problems. A camera white balanced in tungsten light (3,200 Kelvin) will give my complexion a blue tint if I am then lit by the sun through the window. Alternatively a camera white balanced in daylight on an average day (around 5,600 Kelvin) will make me look like a red lobster if you switch on any tungsten lamps.

Well maybe looking a little sun burnt might make me look healthy but with blue skin I'll look deathly. Getting skin tone to look accurate on screen is very important.

To over come the problems of mixed light a colour correction gel is used to alter the Kelvin output of the different light source. It is usual (because it is often quicker) to try and alter tunsten lights to match daylight. A blue gel over a 3,200 Kelvin light source will ensure it (pretty much) matches sunlight at 5,600 degree Kelvin. But, it is also possible to use an orange gel over windows to alter sunlights to match indoor tunsten light - you just need more time and plenty of gaffer tape.

To make white objects appear white to the camera we do a WHITE BALANCE. To do a manual white balance the camera must be shown something white (usually a piece of paper) lit by the light source you will be working in. Beware of white balancing in the wrong light source e.g. by a window and then shooting the interview in a corner of the room lit by tungsten light. Whatever light falls on your subject should also fall on the white paper you use to perform a white balance.

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WHEN SHOULD YOU PERFORM A MANUAL WHITE BALANCE?

• In mixed light (e.g. in a tungsten lit room with daylight coming through a window).
• When shooting subjects lit by fluorescent light
• If light conditions are changing quickly (i.e. at sunrise and sunset when you want the shots to match).
• Every time you move from one source of light (e.g. outdoors) to another source (i.e. indoors).

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SETTING THE WHITE BALANCE

There are four white balance modes to choose from:

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In reality things are never as clear cut as they seem...

AUTO WHITE BALANCE	MANUAL WHITE BALANCE
		These picture were taken in my dining room - which is lit by tungsten light. On my computer monitor the manual white balance looks a little blue. While the auto white balance looks spot on. Preset 5,600 Kelvin looks orange - as expected. While the preset 3,200Kelvin looks OK. But in this case I think I prefer the auto.
PRESET 5,600K (SUN SYMBOL)	PRESET 3,200K (LIGHTBULB SYMBOL)

AUTO WHITE BALANCE	MANUAL WHITE BALANCE
		These pictures were taken in my garden. Between the auto and the manual white balance I prefer the auto. The preset 3,200 Kelvin has made the sky look very blue - as expected. But in this shot - it looks pretty good. The preset 5,600 Kelvin setting is probably more natural and closer to the real thing.
PRESET 5,600K (SUN SYMBOL)	PRESET 3,200K (LIGHTBULB SYMBOL)

HOW TO DO A MANUAL WHITE BALANCE

1. Set the AUTO LOCK switch to the middle position.
2. Press the WHT BAL button
3. Turn the SEL/PUSH EXEC dial until the symbol will appears in the viewfinder/LCD.
4. Point the camera at a white piece of paper and this time press the SEL/PUSH EXEC dial. The symbol will flash rapidly while the camera is calculating the correct white balance.
5. When the symbol stops flashing the white balance is complete.
6. If the symbol continues to flash slowly - try again - try again in auto iris mode - or go to auto white balance mode.

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TROUBLE SHOOTING

HELP! The camera won't white balance manually...

There are several possible reasons why your camera won't white balance....

Are the surroundings too bright or too dark.

If it is very bright the camera is getting too much information - if it is dark, not enough. If you have control over lighting conditions - change them. Otherwise try auto exposure and then a manual white balance.

 

You're under or over exposed in decent lighting conditions

Similar to the above - try auto exposure and then a manual white balance

 

The ambient colour temperature is too high or too low for the electronics to jiggle around with.

To make a decent attemp at a white balance the camera needs light that has a smattering of all the colours in the visible spectrum. Again try auto white balance.

Just a thought... If you try and white balance under sodium lighting you'll probably fail. Sodium street lights are practically a big spike of orange light and not a lot else. In this case either....

1) go to auto white balance

2) select the tungsten preset (the little light bulb symbol) - remember it's probably getting dark anyway (otherwise why are the street light on?) so the pre-set white balance will be OK because the majority of light around will be artificial anyway.

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Move on to...

Part 5: THE SHUTTER

Or back to...

Part 3: EXPOSURE AND GAIN

Christina Fox

> Should a white card be pure white ? I scanned the card and it appears to have a slight blue tint to it.

It should be as white as possible. But, grey will do too - because grey is just "dark white".

For more in depth stuff on white balance I highly recommend the excellent piece by Ex BBC cameraman Tony Grant and our own piece on white balance settings under different conditions.

If you want to do some cheap effects you can white balance on almost any colour. Take a look at our examples. Just experiment and see what you get. I was taught not to white balance on clothing because washing powders have optical brightners that can skew your white balance. Remember "Daz gives you bluey whiteness".

>>>>WHEN do we need to white-balance anyway?<<<<<

a.. In mixed light (e.g. in a tungsten lit room with daylight coming through a window).
b.. When shooting subjects lit by fluorescent light
c.. If light conditions are changing quickly (i.e. at sunrise and sunset when you want the shots to match).
d.. Every time you move from one source of light (e.g. outdoors) to another source (i.e. indoors).

Remember, continuity isn't just about the set and wardrobe - you need consistency in picture quality too.

>>>>>Should I white balance before every shot?<<<<<<

Not really. But, for example - If all your shots are outdoors then one white balance would probably do. But, if you shoot some stuff indoors then go outdoors - you'll need to do a WB for each location.

>>>>Only if its dark or bright?<<<<

If it is really dark then the only light around will be artificial light in this case the "lightbulb setting" (3200K) can work. Otherwise the auto WB will be ok. Most night time shots will look a bit orange due to sodium street lights. Sodiums are pretty much a big spike of orange light.

Brightness doesn't effect WB that much. Although shade and full sun do have different colour temperatures (shade is bluer). On bright days I WB in the shade and it helps to warm up the pictures in full sun.

Outside mainly? Funnily enough I think indoors is trickier. Fluorescents are a pain because they can come in all sorts of colours. Plus, there is almost always mixed light indoors. Which means the area of the room nearest the window will be a different colour temperature to the rest of the room. Nurse - the screens!

Christina