Metering Techniques

Over the last few weeks I have been asked some very interesting questions, relating to colour temperature and flash durations. It seems worth clarifying so that you can see just how important they both are, especially if you are thinking of buying flash for the first time.

Inside the studio there aren't that many problems with colour temperature, flash tubes are 'coated', which is why they look slightly amber. This coating is important, a clear flash tube would fire at around 6400 K and would appear cold and give a blue colour cast, the coated tubes are colour balanced to 5600 degrees Kelvin, standard daylight. So when we use any daylight film, or set digital cameras on auto white balance, we don't need to add filters to correct.

Also the flash duration of any flash head, is a key specification to maintain consistent colour temperature and manufacturers achieve fast flash durations by using high speed tubes on some of their models. All the major manufacturers of flash equipment measure the flash durations of their units in the same way. It is often quoted at T=0.5 and to make this a little easier to understand I have shown a diagram below.

As you can see from the diagram, the flash duration is measured at 50% from its maximum power, and then it's measured again at the 50% point, as it falls away.



It is this measurement, quoted in hundreds or thousands of a second that will tell us what the flash duration of a flash head will be.

As you can see the light reaches its 'peak' power almost immediately and then the 'tail' falls away more slowly. So why is a fast flash duration important?

Well, a flash head with a faster flash duration will have less variation in colour temperature than a unit with a slower measurement.

This is because the 'tail' on the diagram will be shorter on the unit with the faster measurement and longer on the unit with the slower measurement. The tail on the diagram will tell us if we will get a variation in colour temperature.

As all T=0.5 measurements of flash heads are taken at full power, you must expect a little variation of colour temperature when any unit is turned down from full to say half or eighth power. The difference can be measured with a colour meter. It will normally only be 200 to 300 K and will not make a significant difference, but must be taken into account.

The other advantage of buying a flash head with a faster flash duration is that it will help you capture movement. The current vogue for portraiture, high key with people jumping and leaping about is very popular at the moment and I'm always being asked questions about that style. The key to this photography is to have the right studio flash equipment, flash heads with fast flash durations. Unfortunately many people end up buying completely the wrong thing and waste a lot of money.

If you want to do this type of work, buy something like a Bowens Esprit Gemini 750+ it has a very respectable flash duration of 1/2380 of a second and designed for that application.

There are some other obvious things that will affect colour temperature, using an old soft box for instance is a classic. You don't realize just how dirty and discoloured the front covers get, so change them when they go slightly yellow.

The flash tubes themselves will also change colour temperature as the amber coating burns off, but it does take a long time so don't panic.

So to sum up, the type of flash tube a manufacturer uses will give a flash head a normal or fast flash duration and with the amber coating as standard be completely compatible with daylight film or digital cameras. A head with a faster flash duration will be more consistent in maintaining colour temperature, even with the power turned down and help you capture movement. I hope that you find this useful in helping you decide what type of flash to buy.

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