auto-fp flash power tests .

[DesmondD]

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 .


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[DesmondD]

[DesmondD]

Ok well I've been doing more tests and calculations on the subject to try and figure out why we lose so much power as we start going into auto-fp mode and think I just worked it out !
Auto-fp flash logically must stay on for longer for slower shutter speeds .
Let's take 1/1000th sec first , the flash must start 'pulsing' before the first curtain opens - then it must last for 1/280th sec [ if that's how fast the curtain moves ] but once the first curtain reaches the other side it must stay open for the duration of the slit that still has to close !
At 1/1000th sec let's say that slit is 1/4 of the frame = another 1/1120th of a second.



Now we look closer at the slit at speeds closer to normal sync .
1/320th shows a much larger slit [ 3X the size of 1/1000th?]
Now after the first curtain moves across the frame [ 1/280th sec] the flash still has to remain on until the second curtain is closed which will now take 3X as long as at 1/1000th = 3/1120th = 1/370th sec extra !



This would explain why the first drop into auto-fp mode gives us so much less power than 1/2000th .
When I fire the SB24 at 1/16th on the D90 at 1/250th sec I don't see the curtain starting to close yet though sync speed is back at 1/200th .
This means that when the flash goes into fp mode it is doing so over 1/280th sec plus the 1/250th sec that the rear curtain takes to close cutting the total power in half[ approximately ] .
So auto-fp would have to push out around half as much power over twice the time as opposed to around 1/250th sec shutter travel at 1/4000th sec - not counting the power before and after the pulse .



So essentially different shutter speeds would vary how long auto-fp stays on , and how much power it pushes out over that time period resulting in the differences we see between 1/250th and 1/2000th in auto-fp mode !

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[DesmondD]

Tonight I made up a graph of a purely 'theoretical' camera with a shutter curtain speed of 1/250th sec to make the concept easier to understand .



We take a picture at 1/250th sec . The shutter is fully open before the second curtain starts to close .


Once the first curtain reaches the bottom [ after 1/250th sec ] , the second curtain starts to close and also takes 1/250th sec to complete its journey - a total shutter travel time of 2/250th sec / 8ms .

When we go to 1/500th , on the 'theoretical camera' this slit will be exactly half of the frame ....



The first curtain takes 1/250th sec [4ms] , the second curtain is already halfway across so it takes half of 1/250th sec [1/500th ] to finish its travel[2ms] for a total travel time of 6ms .

At 1/1000th sec the slit will be 1/4 of the frame :



1/250th plus 1/1000th sec total shutter travel time .

At 1/2000th sec the second curtain will take 1/250th plus 1/2000th sec to complete its exposure ..... at this point there isn't much change as the speed increases .

Now what is interesting is that while I was producing my theoretical shutter curve Pete posted a link to his blog where he has done some actual tests .... interestingly the pulse duration of a D90 and SB800 at 1/250th sec is a little over 8 ms .....
At 1/2000th it's a little over 5 ms ....
It would appear that the D90 has been programmed to be able to select its fp pulse time for each shutter speed - not hard to imagine with today's technology , in fact it would be strange to find out that they hadn't done this with today's technology .




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