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How-to's

When is Full Power Flash not Full Power?

Russ MacDonald (Arkayem)


Keywords: nikon, speedlights, lighting, flash

When you set your flash in Manual mode and full power, you expect to get a full power flash, right?

Well, you would be wrong! At least part of the time.

Many of you will be surprised to learn that even with your flash in Manual Mode you will only get full power when the shutter speed is slower than about 1/125th sec!

This is because when the flash is in Manual mode and mounted on the hot shoe, it turns ON and OFF with the Sync pulse that fires it through the center terminal.

 

 


When the flash truly fires at full power, like when you hold it in your hand, it completely discharges the capacitor, and it takes longer to do this complete discharge than the time available at the flash sync speed of 4 ms (1/250th sec).

In fact, the tail of the flash pulse goes out to about 6 - 8 ms before it is completely gone. This portion of the tail is well below the half power point that is used in the specification to define the length of the full power flash. This is how they get by with saying the flash is 1/1000th sec when at full power. That's the length of time between the half-power points on the waveform.

So, when the flash is in the hot shoe, the flash fires when the Sync signal is applied as the shutter opens, and it is quenched when the Sync signal goes away as the shutter closes. This stops the complete discharge of the flash if there is any energy left in the capacitor.

Now, when the shutter is at 1/60th (17 ms), the flash has plenty of time to completely dump all the charge on its capacitor. However, at faster shutter speeds, like 1/250th (4 ms), there is still significant charge left in the capacitor when the flash is squelched.

This remaining charge is also why the flash will recycle faster when at 1/250th shutter than when at 1/60th, even when the flash is in Manual mode and set to 1/1 (full power).

 

This also explains why the image from the 1/60th shutter is brighter than the image from the 1/250th shutter when the flash is fired at full power. The tail that is cut off at the higher shutter speeds but present in slower shutter speeds, contributes to the exposure.

20130909_085454_15.image-1_when_is_full_power.jpg

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11 comments

Steve White (boogie34950) on January 1, 2017

Thanks

Narikunni Jayanth (jaydoc40) on September 16, 2013

Thanks for the detailed information. I noted that When I dial in +0.3 ev the picture details show that I have put +3ev. But when I dial in +0.3 plus flash power and shoot, the picture details show +0.3ev with +0.7 flash power.Why is it so?

Jose Santos (espeto68) on September 15, 2013

Thanks for the info Russ.

Alan Martin (GeorgCantor) on September 15, 2013

Excellent eplanation, Russ. Why is there a Photo Ninja ad in the middle of your posting? Is that from you, or is it from the Nikonians org? This is the first time that I have seen an ad in the middle of a posting like tihs.

Robert Kim Holwick (hillsidekim) on September 13, 2013

Thanks again Russ.

Richard Luse (DaddySS) on September 11, 2013

Ribbon awarded for his generous support to the Fundraising Campaign 2014 Ribbon awarded for his generous support to the Fundraising Campaign 2015 Ribbon awarded for  his generous support to the Fundraising Campaign 2017 Ribbon awarded for his generous contribution to the 2019 Fundraising campaign

Interesting and good to know, thanks Russ.

User on September 10, 2013

That's fascinating. It explains why some of my results didn't come out as expected.

RAJENDRA M. PANDIT (RMPANDIT) on September 10, 2013

Thank You Russ for important Information,I still keep learnning at my old age.

David Young (davepmyoung) on September 10, 2013

Even at my ripe old age you can still learn new things, I never new that! Thanks

KENNETH JACKSON (f5titan) on September 10, 2013

I used Nikon F3's for 18 years and at the 1/80th second sync speed this was never a concern. When I acquired the F5 I noticed a bit of exposure loss at 1/250th second so I never used my SB80 with any sync speed over 1/125 second. Now I know why! Thanks for the information.

Mike Banks (unclemikey) on September 9, 2013

Russ, I never thought about this even though I know the math. Thanks, this explains a lot regarding an event I just shot with inconsistent results in Manual Mode with the SB-910. Keep em coming!!!

G