Bursting Balloons !

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Friday, August 27, 2010

auto-fp flash power tests

This afternoon I did some tests to compare the power of auto-fp to normal flash . I used a solid white wall and kept the flash set at 1/1 full power and also made sure I kept the ambient level [which  was very under-exposed ] the same , to simulate a situation where we would keep our ambient settings in the same place while reverting to auto-fp mode .
I aimed to get the same histogram in each shot and it was interesting how much difference a few cm made when I made adjustments .

This is what I aimed for :

After each change in settings I moved the camera until I got the same histogram and then measured the distance to the flash head so I could calculate the power difference compared to normal flash .

F25        1/200th normal flash               195cm
F22        1/250th auto-fp mode             68cm  =  1/8th of normal flash power
F16        1/500th                                88cm =    1/5
F11        1/1000th                              120cm =  1/2.6
F8          1/2000th                              136cm = 1/2
F6.3       1/3200th                              120cm = 1/2.6
F5.6       1/4000th                              112cm = 1/3

My conclusion is that due to the way auto-fp works even though the flash head suggests the same working distance at any of these settings when in auto-fp mode it appears that the combination at 1/2000th sec yields the greatest output , after which the curve drops off again .
Since auto-fp flash behaves as a continuous light the slower the speed the more energy it requires ! So faster shutter speeds mean that light is 'switched on' for shorter times , not counting the short period before the shutter opens and after it closes that it will need to be on . This should also result in faster charge times .
In other words , when you revert to auto-fp mode stay away from the the slower speeds and aim for 1/2000th sec and the aperture that gets you the results you want to get the most flash power possible in this mode .

The results :

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