How Hot is the Light?
The Effect of Colour Temperature
The temperature of light how warm or cold an image appears to be, is something that photographers have always had to be concerned with. Many people find the concept difficult to put into practice, but with a good understanding of the theory, and with the new possibilities to experiment through the use of digital capture, light temperature is something everyone can easily understand and use to their advantage.
Colour temperature is measured in Kelvin and it is possible to buy colour temperature meters, however, these are fairly expensive and for the most part, not strictly necessary. The table below shows the colour temperature values of some common lighting situations and is sufficient as a guide for most photography.
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Temperature |
Typical Sources |
Filter |
Converts to |
| 2500-2800 | Household light bulbs. | 80A | 5500 Normal Film |
| 3000-3200 | Tungsten Halogen typical studio lights. | 80A/B | 5500 Normal Film |
| 5000 | Typical Daylight. | ||
| 5500 | Daylight mid-day, sun
overhead. Flash. |
85B | 3200 Tungsten Film |
| 6000 | Very bright, direct sunlight. | ||
| 8000-9000 | Shade | ||
| 11000+ | Very overcast conditions. |
Table 1 colour temp values under different lighting and correction filters for film.
You can see the effect of shooting the same subject whilst differing the effective colour balance of the capture medium in the following images. For this example, a Canon EOS10D was used, and the various colour temperature points have been set using software. At 3200K and 5500K, you can still observe the effect that you would see using normal tungsten and daylight balanced film:
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3200K (= image that would be captured using Tungsten balanced film) |
4100K (The level the camera selected with the white balance set to Auto) |
5500K (= image that would be captured using normal daylight balanced film) |
7150K (You would see this type of result using daylight film and a warm up filter) |
8550K (And this if you had used too strong a warm up filter!) |
Clearly the correct colour temperature lies somewhere around 7000K, which compared to the previous table of temperatures, is about right for bright shade. The daylight-balanced setting gives a good result, but a weak warm up filter would have been the right choice with film.
By having an awareness of colour temperature, it is possible to pre-visualise how your image will be recorded on the medium you are using, and you can choose to either compensate, or use the light to creative effect.
COMPENSATING FOR THE TEMPERATURE ON FILM
How to compensate for or correct colour temperature is dependent on whether you are shooting film or digital. Film users have two possible starting points either daylight or tungsten balanced film. Daylight balanced film is balanced for light at 5500K and tungsten is balanced for light at 3200K. This means that if you were to shoot a scene, lit by unobstructed sunlight (i.e. no cloud) on daylight balanced film, it would accurately record the colours you see.
The same applies to the tungsten-balanced film in a scene lit purely by tungsten lighting.
Without correction, under lighting that the film was not designed for, a colour cast will result. Most people will recognise orange-looking images shot on normal daylight balanced film under normal indoor tungsten lighting.
It is possible to use this knowledge of the medium to creative effect by purposefully using a film with the wrong colour balance:
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Captured in a 10D, which chose 7100K on Auto. |
Set at 5500K, this is how the image would have looked if captured on Daylight balanced film. |
Set to 2800K, by far the best effect would have been captured using Tungsten balanced film. |
Tungsten can often produce a nice effect when used at dusk, just after the sun has fallen below the horizon.
If the aim is to accurately record the scene as it really is and the light temperature is not at exactly a daylight or tungsten level, then film users must use colour correction filters. Table 1 also shows the filters necessary to compensate for various lighting situations when using daylight and tungsten balanced film. The key to this game is being able to judge what the light temperature actually is, in order to be able to correct it. This is a skill that pro photographers have built up over many years of experience. Even so, if the image is of particular importance, most people will choose to take a number of images with various strength filters to be sure of obtaining the desired effect. Slightly over-compensating will also give a warm effect something often applied to landscape images.
THE BEAUTY OF DIGITAL
One of the features of digital capture that is exciting many photographers, is the ability to change the colour temperature in the camera. Most digital cameras allow at least some, if not full control over the colour temperature recorded. On basic models, it can be set to auto, or to some fixed light temperature points such as sunlight, cloud, tungsten etc. More advanced models allow auto or full manual control in 100K or so steps, meaning that there is no longer any need to buy or carry around any colour correction filters!
With a little practice, guessing the colour temperature soon becomes second nature. It is even possible to fairly accurately estimate the combined colour temperature from two sources that have different temperatures. In this shot, there is a mixture of flash (almost the same temperature as daylight) and tungsten lighting in the room. The temperature was manually set on the 10D to a value between that of the two sources (4450K) to give natural looking colours and to be able to set the exposure and benefit from all the available light.
If your camera offers less control, you can still manually apply the fixed-point settings for example applying a sunny temperature setting to warm up a cloudy scene. Whilst the auto settings of digital cameras can be effective, better results can often be obtained by interpreting the light temperature manually and the beauty of experimenting in the digital medium is that other than some memory card space, its free!
An alternative method of setting the colour temperature on some digital cameras is in effect, not to set it at all. By saving images in the RAW mode, the raw data from the sensor is recorded. In digital cameras, the colour temperature effect is applied by software, therefore by saving the RAW data, this processing is not done in the camera. The camera suppliers (or third party) software can read the RAW data and the colour temperature can then be applied by the computer. This means that you can change the colour temperature of the image across the full range and decide on which is the most pleasing or accurate rendition! On the computer you can almost take one aspect of the image as many times as you like to get the desired effect all from the comfort of a chair in your own home! Additionally, this means that when you are out shooting, you need not be concerned with the colour temperature at all you can simply focus on the composition and exposure and choose the colour temperature later on the computer. This is a very powerful feature of digital photography and is one of the factors that will ensure that the uptake of digital continues.
Words and images copyright Steve Pearce 2004.
