Measuring the flash or burning time
In macro or microphotography it is crucial among other factors to work with very short exposure times.
I use the term “t0.5” as a measure for the burning time of a flash. That is the time taken from the 50% maximum until the value has dropped down to 50%.
Less convenient are photo-transistors for measuring, since its rise time and decrease times are approximately 5-15 micro seconds. Phototransistors have to be connected to an independent power supply unit.
More useful is a photo diode which rise and decay time is approximately 5 nano- seconds. The Photodiode in parallel can be connected, with a 10 Ohm resistance to the microphone plug of a sound card on a computer and thus the curve can be visualized and measured in a audio program (e.g. the freeware audacity).
It must be pointed out that the flash is not placed to close to the photodiode; a distance of
2- 4 m is recommendable. In case of a too close position the diode is going to be overdriven and the curve shows a flat maximum; in addition, besides the light time also the voltage is
measured. The entry level of the microphone should be set to approximately 80% to avoid an overdrive as well. Furthermore, one has to keep in mind that the sound card and the software digitalize the analogues signal with a sampling frequency of 96 kHz and have to be appropriately adjusted.
A correctly measured curve, at full power, is characteristic, showing a fast rise to maximum, a roundish top and a slow decrease in an e-function manner.
Using reduced power, the curve shows after some time a sudden drop and if the power of the flash is even more reduced the drop in the curve occurs before the maximum output is reached (voltage and light intensity).
This method allows measuring and comparing different flashes.
This method of measuring the flash burning time is simple and cheap. However, it is to keep in mind that the measurement is not absolutely precise! But it is an appropriate way to compare flashes in comparable circumstances. The time measuring point of t0.5 (half voltage or -6db) can only be found roughly, since the curve (of the photodiode, the sound card and the software) is not absolutely linear in its amplitude.
The photodiode I am using is:
SIEMENS Silicium-PIN-Photodiode with short circuit time SFH 203.
The approximate price is 1 euro.
My measurement comparisons results are (these are not obligatory and absolute numbers!)
Nikon Speedlight SB-16 (in position "MD", the burning time is approx. t0,5 = 1/10’000 s.).
Nikon Speedlight SB-800 (at 1/128 power, the burning time is approx t0,5 = 1/33'000 s.).
YONGNUO Speedlite YN560-III (at 1/128 power, the burning time is approx t0,5 = 1/20’000 sek.).
SUNPAK auto 16R pro, Ringblitz (at 1/256 power, the burning time is approx t0,5 = 1/33’000 s.).
Studio flashes possess a high guidenumber (e.g. 55), however, the burning time usually is about the same over all power levels (full to 1/32 power), typically the burning time is approximately 1/200s to 1/1'000 s, due to long flash bulbs. The power reduction is regulated by the light intensity and therefore studio flashes are not suitable for macro photography.