FP (or HSS) mode v. higher sync speed

Started Aug 10, 2012 | Discussions thread
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dbbarron
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FP (or HSS) mode v. higher sync speed
Aug 10, 2012

I've been thinking a great deal about using FP sync mode versus using a camera with a high sync speed (I have an old D70) along with a speedlight (or even studio strobes for the higher sync speed). My goal is maximum light output in either configuration (FP sync or high speed shutter sync).

In the case of FP mode, it would seem that by 1/8000 shutter speed you effectively lose 2 stops of light (f/4 at 1/8000 v. f/8 at 1/250 as measured by a flash meter using a SB-800 on a D7k).

I initially thought that switching to a camera with a higher sync speed would fix that, but I was wrong. I failed to account for the full power flash duration being relatively long as compared to shutter speeds. Thus, the flash duration must be shorter than the shutter speed in order to get all the flash light in the exposure. Thus, for 1/8000 shutter speed, I need a duration of shorter than 1/8000. On a SB-910, the nearest power level to achieve this is 1/10000 duration at 1/16 power. That is roughly the same as the 2 stop FP loss.

Thus, it appears there is no advantage in light output as between FP and higher speed sync using a speedlight. Of course, there are other advantages & disadvantages with each depending on the circumstance and what shutter speed you want to use.

Thus, it appears that even if you have a camera with a fast sync speed, the only way to get higher flash output is to switch to a much bigger flash, e.g., monolight(s) (which again presents flash duration issues) or use multiple on camera type strobes.

Have I made any errors with this analysis? Have I missed a configuration that will permit greater flash power at either normal sync or high speed sync shutter speeds.

db

Nikon D70
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