While chatting to a few people at IBC it was brought to my attention that some people find my blog a little too advanced and intimidating. So in order to try to address that I’m going to go back to basics and write about some of the basic principles of digital video. The plan is to look at some basic concepts and then expand on these so that even beginners will gain a deeper understanding of these topics.
To kick off I’m going to look at White Balance.
What Does it Do?
Accurate white balance ensures that white objects in your video appear white, in addition it ensures that colours look natural and similar to the colours you see with your own eyes while shooting the scene.
How do I set it?
You can use the cameras auto mode, often called ATW (auto tracing white balance), a preset white balance, calibrate the white balance using a white or grey object or manually dial in the white balance.
Different light sources have different colour temperatures. For example an incandescent light bulb, the type with a glowing filament has a low colour temperature while an arc lamp which is much hotter will have a higher colour temperature. Low colour temperature light sources like a filament light bulb are at the red or orange end of the visible light spectrum while hotter light sources like an arc lamp or the sun are at the blue end of the light spectrum. Our own visual system will adapt to these varying colour temperatures but a video camera needs to be set to the correct colour temperature to reproduce colours correctly. The colour temperature of a light source is measured in “degrees Kelvin” or “K”. Lower numbers will be at the red end and higher numbers at the blue end. So a filament light bulb will glow orange at typically 2,300k while the mid day sun will be around 5,500k and the light from a clear blue sky is around 9,000k. If you shoot with a camera set for a low colour temperature i.e. 3,200k outside on a sunny day your pictures will be very blue. If you shoot with a camera set for a higher temperature like daylight, typically 5,600k under filament lamps the pictures will be very orange.
Different Types of White Balance Setting.
Full Auto, ATW, Auto Tracking White, Auto White: This is generally a fully automatic mode that tries to guess the correct white balance. It normally works by assuming that the brightest parts of the scene are white objects and will try to correct these objects so that they appear white. As a result ATW is often easily fooled especially by bright slightly off white walls. As ATW is always active if the scene you are shooting changes then so may the white balance and this can be seen as a colour shift during the shot. ATW can be a little unpredictable and trying to later correct a shot where the white balance changes part way through can be very difficult. One advantage with ATW is that if you move from one place with one colour temperature to a place with a different colour temperature, for example moving from an interior room lit by filament lights to outside daylight then the ATW should compensate for the change in colour temperature automatically. On many pro cameras ATW can be assigned to the white balance “preset, A, B” switch “B” position.
Preset White: Preset White allows you to pre set the colour temperature manually. Normally this preset will be for 3,200K (Tungsten filament lamps) or 5,600K (average daytime exterior). Many more recent Sony cameras will have a switch to select preset white and then a further button to alternate between 3,200 and 5,600K. Preset white is useful when you have a multi camera shoot to get all cameras matching or for setting the white balance when shooting under coloured lights such as stage or concert lighting where ATW would really struggle or manual white balancing may give an incorrect colour balance. It’s is also often the white balance method of choice on projects that will be colour graded in post production as it gives a constant setting from shot to shot, scene to scene which colourists often find easier and faster to work with.
Manual White Balance: This is sometimes misunderstood, but when done correctly will give the most accurate results. This is where you have a 3 way switch marked “Prst” (preset) “A” and “B”. When in the “A” or “B” position (assuming ATW has not been assigned to the “B” position) a white balance memory is used to determine the colour balance of the camera. To get the correct colour balance the camera needs to be sample and memorise the colour temperature of the light in the scene you want to shoot. This is done using a white or grey card or object. It doesn’t really matter which you use provided the card or object is not coloured in any way and can be correctly exposed. Be careful what you use, a lot of paper is dyed slightly blue to make it appear brighter. If you have a choice the best thing to use is a grey card designed for photography or video exposure and white balance. Place the card at the center of your shot, you want the primary light source for your scene to be falling on the card. Point the camera at the card, it should fill at least 50% of the frame and expose it so that it is around the middle of your exposure range, not bright white and not dark grey, right in the middle. Then with the white balance switch in the “A” or “B” position press the “white balance” button (often found on the front of the camera under the lens or close to the white balance switch). The camera will then adjust the gain of the red, blue and green channels so that the grey/white card shows no colour and will often display the measured colour temperature in the viewfinder or a simple “OK” message. If you get a “NG” or error message you may need to adjust your exposure up or down a little. Look at the pictures on a colour screen and check that they look OK. If not, check that the grey/white card is in the centre of your frame again and try repeating the process. You can store two manual white balance settings one in the “A” memory and one in the “B” memory and switch between them as needed.
Advanced White Balance Techniques: Sometimes it can be useful to use a white balance colour temperature that may be different from the actual colour temperature of the scene. For example if you are shooting a sunset the colour temperature of the actual light might be around 2,000 to 3,000K. If you were to white balance the camera to match this it would neutralise the orange glow of the sunset making it much less colourful. So to shoot a sunset you might want to have the camera set to a higher colour temperature to accentuate the orange sunset light. Setting the camera to preset 5,600k might work, but perhaps this takes things too far and makes it too orange. In this case you might want to dial in an in between value for your white balance like 4,400K. Some cameras will allow you to do this by changing the preset white value (in picture profiles or scene files on most Sony cameras). Alternately some cameras will allow you to add an offset to your manually set white balance, but do remember if you do this to remove it again once your done with it.
Another way to white balance the camera with a warmer or cooler white balance than the actual lighting conditions is to use a coloured card instead of a white/grey card. If you use a card with a slight blue tint then the camera will think the scene is bluer than it really is. This results in a final image that is warmer (more orange) than if you had used a white/grey card. If you use a card with a slight orange tint then the camera will think the ambient light is warmer resulting in a final image that looks cooler (more blue). Instead of using coloured cards (often called warming or cooling cards) you can achieve exactly the same effect by doing your white balance through a blue or orange lighting gel or filter.
What to avoid: Try to avoid mixing different light sources with different colour temperatures. For example a person sat close to a window, lit by a filament light will be illuminated by the orange 3,200K light from the lamp as well as the blue 5,600K light from outside. This can result in some strange colours and the scene may look odd (of course you may deliberately choose to have this colour contrast to enhance your scene). Some light sources contain peaks and gaps in the light spectrum that they emit and this can result in odd colour reproduction. A common example of this is fluorescent lights used in homes and offices that often have a pronounced green tint to them that white balancing alone will not remove.