HSS Without TTL?

Started 2 months ago | Discussions thread
T Evergreen Regular Member • Posts: 109
Re: HSS Without TTL?

Adam007 wrote:

Evergreen,

I don't follow you. I understand that the flash generated by TTL is...whatever power level the flash determines is needed - 1/1, 1/2...whatever. My issue is that the pre-flash TTL requires robs me of recycling time. Admittedly, I have never run a side-by-side test of TTL and manual flash recycle rates. But my experience is: longer recycling times once you get beyond a couple flashes. I will often be using this beyond a couple flashes.

Now if you're saying that the pre-flash uses very little power - and thus doesn't basically mean you are flashing twice for every exposure, that's news to me. I've always assumed the pre-flash was at full power, but I don't know.

Your data on the loss of power from HSS is dramatic. Which reaffirms for me that I can't afford to lose anything (time, basically) by using TTL on top of that.

The data I cited earlier is not my data, rather it's Pentax data from its owners manual.

Regarding the pre-flash in P-TTL, there is only so much "juice" in the flash's capacitor and if the "juice" were depleted by firing full power for the pre-flash, there would be nothing left to light the scene until the capacitor were once again fully charged.   I have no specific knowledge about how powerful the pre-flash is, but it can't be very much because the maximum reach of the flash is the same whether using manual flash or P-TTL according to Pentax.

I'd expect that the recycle rates for P-TTL and Manual to be identical IF both put out the same amount of light when firing.  The problem, however, is that you don't know exactly how much light is being put out when in P-TTL mode.

When shooting at sync speed and below, one flash from the speedlight is enough to fully illuminate the sensor.  However, as was mentioned by another poster, HSS requires multiple flashes precisely timed to illuminate the sensor.   As you go to higher and higher shutter speeds, you need more and more flashes and that is why the Guide Number for the flash decreased as the speed of the shutter increases.  At 1/8000 second the flash needs to fire so many times to illuminate the sensor fully that the maximum Guide Number for the Pentax AF540FGZ II drops to 5, which is about the same as using the flash at 1/128 power in manual mode.  Unfortunately, in manual mode at 1/128 power you're using only a very small amount of "juice" while in HSS mode you're using all of the juice in the flash, with a corresponding delay in recycle times.  According to Pentax, it take 3 to 5 seconds to fully recharge the AF540AFG II depending on the type of batteries being used.

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