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Conclusion

• Flash duration varies between 60 and 160 microseconds.
• The peak intensity only changes slightly between preflash and main flash; what changes is the flash duration.
• In the long range image the camera realises that the subject is far away and increases the flash duration.
• In macro mode the camera selects automatically a small aperture.
• However the main exposure control mechanism seems to be the aperture. The flash itself plays a smaller role.

The test

• When using the flash the Olympus 4040 and many other digital cameras fire first a preflash and after an instant the main flash.
• The pre-flash is used to measure the power needed for the main flash. This way the camera can precisely set the flash power without over- or underexposing the image.
• To measure the time between pre-flash and main flash I used a fototransistor (light sensitive semiconductor) connected to the microphone input of my computer's sound card:

Circuit

This is how everything looks like.

• I used Goldwave (a sound recording and editing software) and set a sound file with a length of 1 second. This is the screen I got (resized to 50%):

• The sampling speed is set to 48000 samples/second, i.e. one every 22 microseconds. This way it is possible to measure light intensity changes every 22 microseconds and to store them for further analysis.
• I took two pictures with flash, one at close range (macro) and one at three meters - to get an idea how the flash pulses change.

Three meter test

• Here the flash has two illuminate a subject at 3m distance. This is what Goldwave records:

The preflash comes at 560 ms, the main flash at 676 ms, i.e. the main flash comes 116 ms after the preflash.
 

Short distance test

Preflash at 452 ms, main flash at 574 ms - here the main flash comes 112 ms after the preflash
 
 
 

Flash pulse intensity and duration

(Note: the flash pulse is negative)
 

Preflash 5 meters Duration 60 microseconds, peak 0.045 F1.8, 1/30s	Main flash 5 meters Duration 160 microseconds, peak 0.06 F1.8, 1/30s
Preflash short range Duration 60 microseconds, peak 0.075 F8.0, 1/50s	Main flash short range Duration 60 microseconds, peak 0.06 F8.0, 1/50s

• Flash duration varies between 60 and 160 microseconds.
• The peak intensity only changes slightly between preflash and main flash; what changes is the flash duration.
• In the long range image the camera realises that the subject is far away and increases the flash duration.
• In macro mode the camera selects automatically a small aperture.
• However the main exposure control mechanism seems to be the aperture. The flash itself plays a smaller role.

Back to the Olympus 4040 online resource