Photography Lesson 11 : Colour Temperature ( Kelvin )

Light is not always what it seems. Our eyes are very forgiving, so when we move from place to place everything seems normal in terms of colour. So, for example, when moving from a bright daylight environment to a room lit by a candle all that will appear to change, to the naked eye, is the light level. Yet record these two situations using colour film and the first will have a blue hue and the latter will come out with a heavy orange cast. This is because our brain can quickly adjust to the changes, making white appear white, whereas film is balanced for one particular colour and anything that deviates from this will produce a colour cast.

The colour variation is referred to as the colour temperature and is measured in degrees Kelvin. The scale ranges from the flame of a candle at around 1900K to deep blue sky at around 10,000k as illustrated by this diagram.

The light source is listed next to the degrees in Kelvin, both set against the colour of the light at each level.

You will notice that the colour next to the Average noon daylight (5500K) is white. This is the colour temperature that colour film is balanced to which means that when shooting using daylight film the photograph will record white objects as white and all the colours in an image will appear natural on the film. If you shoot in conditions that measure a higher temperature the photo will start to become bluer and shoot in conditions below the 5500 temperature and they will increase in orange.
The colours here don't just relate to daylight conditions artificial light also introduces a colour cast. With fluorescent lights this is often green and with tungsten lighting it will be yellow, while flash can be slightly blue.

If you own a camcorder or digital camera you may have read about the white balance control. Most digital cameras have an automatic colour balance. This will look at a scene and if it thinks the colour is too blue it compensates to make it natural by adding a red shift to the colours recorded. Likewise if it sees an orange scene it will increase the blue content. This

With film cameras you would have to use colour correction filters to compensate.

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Colour control filters are broadly split up into two categories: colour conversion and light balancing filters. The strong colour conversion filters are in the 80 and 85 series and are used for large colour changes while weaker lighting balance filters fit in the 81 and 82 series and are used for making small colour adjustments.

Film	Kelvin	Illumination	Kelvin	Filter	Colour	Exposure
Daylight	5500K	Tungsten House lights	3200K	80A	Dark Blue	1 1/3
Daylight	5500K	Tungsten Photofloods	3400K	80B	Dark Blue	1
Daylight	5500K	Tungsten Clear flash bulbs	3800K	80C	Dark Blue	1
Daylight	5500K	Daylight Shade under blue sky	7500K	81EF	Straw	2/3
Daylight	5500K	Daylight Shade partly cloudy sky	7000K	81D	Straw	1/3
Daylight	5500K	Daylight Shade under daylight	6500K	81C	Straw	1/3
Daylight	5500K	Daylight overcast	6000K	81A	Straw	1/3
Tungsten A	3400K	Daylight	5500K	85	Orange	1
Tungsten B	3200K	Daylight	5500K	85B	Orange	1
Tungsten	3800K	Daylight	5500K	85C	Orange	1
Tungsten B	3200K	Tungsten lights 100W	2900K	82B	Pale blue	1/2
Tungsten B	3200K	Tungsten Photofloods	3400K	81A	Straw	1/2
Tungsten A	3400K	Tungsten lights 100W	2900K	82C	Pale Blue	2/3
Tungsten A	3400K	Tungsten Clear flash bulbs	3800K	81C	Straw	2/3

The table above shows the film you are using in column 1 and the colour it's balanced to is in column 2. Column 3 is the light conditions you are shooting in and column 4 is the colour temperature of the light. Column 5 is the filter necessary to make the photo look natural and column 6 is its colour. The final exposure column shows the exposure increase that's necessary to adjust for the strength of the filter. This is automatically adjusted by the camera's through-the-lens metering.

Some typical color temperatures are:

1500 K		Candlelight
2680 K		40 W incandescent lamp
3000 K		200 W incandescent lamp
3200 K		Sunrise/sunset
3400 K		Tungsten lamp
3400 K		1 hour from dusk/dawn
5000-4500 K		Xenon lamp/light arc
5500 K		Sunny daylight around noon
5500-5600 K		Electronic photo flash
6500-7500 K		Overcast sky
9000-12000 K		Blue sky

The other type of light you will come across, usually in offices, factories and kitchens, is the fluorescent tube which gives off a green colour when daylight film is used. To correct this you need to use a magenta filter, known as an FL-D (Fluorescent daylight) or FL-W (Fluorescent white) depending on the tubes used. This photograph shows the typical characteristics.

With a digital camera things are far less complicated. Just point and shoot with the camera set to auto. If there's a colour cast when you preview the photograph delete and reshoot using the relevant manual white balance settings. Some cameras also have a fully manual setting where you point the camera at something that should be white. The camera measures the colour reflected from this and adjusts to make the subject white which corrects the colour.

You also don't have to correct colour at all. Rules can be broken. Adding a blue filter when using daylight film can produce landscapes with added mood, especially when shot at dawn or dusk. While the 81 series are often used to warm up skin tones in many situations.

Some situations can be extremely tricky to correct. Take this interior. The window light is stronger than the interior tungsten light. So you get a mixture of colours to worry about. If a blue filter had been added to correct the yellow the stairs and floor would have gone blue. Professionals get round many shots like this by placing huge sheets of lighting gel over windows (yellow in this case) to balance the daylight with the interior light. They then use camera filters to correct the colour cast. I feel in this example leaving the natural colours to affect the film in their own way has worked and I wouldn't want it any other way. Professionals also use colour meters to measure the light acurately. We have a guide to these here. You can often correct a colour cast using your image editing program's channels or colour balance control, but that's another article! Tip The filters can even be used to unusual effect with flash by placing one colour over the lens and a correcting colour over a flash. Then when a photo is taken the flash illuminated subject will look natural while the background will take on a colour cast.

Photography Lesson 10 : Rear Sync Flash

SLOW SYNC FLASH / REAR SYNC FLASH

One camera function that can be a lot of fun to play with (and that can get you some interesting results) is slow sync flash.

Low Light Photography Options

When shooting with a subject in low light situations you generally have two options; either to shoot with a flash or to shoot with a slow shutter speed.

1. Flash - When shooting in low light with a flash in auto mode your camera will choose a relatively fast shutter speed. This means that your subject will be well lit and that if it is moving it will be frozen and as a result will be sharp. The problem with this is that it can also leave your subject lit up too brightly and can leave it’s background looking very dark as there is not enough time for the camera to collect any ambient light.

2. Slow Shutter Speed - The other option is to turn your flash off and shoot with a longer shutter speed in order to collect enough available light from the image to get a well exposed shot. This can be an effective technique if you’re shooting landscape or environmental shots where everything is nice and still - however if you’re shooting a moving subject it means you’ll get motion blur which could ruin your shot.

Both of the above options are legitimate technique but both have their weaknesses. Another options to consider is slow sync flash.

What is Slow Sync Flash?

Slow Sync Flash is a function found on many cameras that tells your camera to shoot with both a longer shutter speed as well as firing the flash. This means you get the best of both worlds above and can both get a relatively sharp shot of your main subject as well as get some ambient light from the background and foreground.

Some cameras allow you to access slow sync flash manually and set exposure length and flash strength but on many compact cameras there is a little less control given and it’s presented as an automatic shooting mode, often called ‘night mode’ or even ‘party mode’ where the camera selects the slower shutter speed and flash strength for you.

Rear and Front Curtain Sync

If your camera gives you some manual control when it comes to slow sync flash you might find yourself presented with two options called ‘rear curtain sync’ and ‘front curtain sync’.

These two modes sound a little technical but to put it most simply they are the way in which you choose when to fire your flash during the longer exposure.

Rear Curtain Sync - this tells your camera to fire the flash at the end of the exposure. ie when you press the shutter your lens opens up and starts collecting light and just before it closes the flash will fire to light up and freeze your main subject (see the card shot to the left for an example where you’ll see the card trail ending in a nice crisp shot of the card).

Front Curtain Sync - this tells your camera to fire the flash at the start of the exposure. ie when you press the shutter, the flash will fire immediately and the shutter will remain open afterwards capturing ambient light.

You might not think there’s much difference between these modes but when you’re photographing a moving subject it can have a real impact. You’ll find many action/sports photographers will use Rear Curtain Sync when shooting with a panning technique.

Tripod or Handheld?

When using either slow synch in either mode (or in the automatic ‘night mode’ you will want to consider whether or not to use a tripod. Traditionally when shooting with longer shutter speeds it is accepted that a tripod is essential in order to stop any camera movement. Even the steadiest of hands will not be able to stop a camera moving over even a 1 or 2 second exposure. So if you want to eliminate blur from your cameras movement definitely use a tripod (and consider a shutter release cable).

However in some circumstances hand holding your camera while using slow sync flash can lead to some wonderful effects. For example if you’re at a wedding or party and are out on the dance floor the results can be great at capturing the mood of a night with those you’re photographing largely frozen by the flash but the lights on the dance floor blurred from you moving your camera during the shot.

Of course hand held techniques won’t work with every situation so experiment with both methods at different shutter speeds and by using both rear and front curtain sync and find the best methods for your particular situation.