"Cumulative" power cuts; Mitros+ speedlites

[ScottyP]

I have recently purchased three Phottix Mitros+ flash units. I have been playing around with them, and a couple of light stands and shoot through umbrellas.

The manual says that the power cuts on the transmitter and on the receiver are "cumulative" to each other. This definitely proves true in my experiments with them so far.

Does this mean that I am basically getting a lot more low-power settings using two units than you could get using just a single unit on camera which would bottom out at 1/128? I Have noticed that sometimes 1/128th power is too much up close. So does using a wireless flash like this really extend the low-power capability of the flash in the way that the math seems to suggest? If so, that is a serious extra benefit to having the two units and the wireless connection.

 

[privatebydesign]

Yes that is correct, two units at 1/128 give you the same output as one unit at 1/64, a stop more light at lowest setting.

Of course in manual you have other options if you want less light, move the flash back 1.414 times the distance to get one stop less, use diffusion etc etc.

 

[ScottyP]

But the power reductions on the transmitter combine with the cuts on the receiving flash to reduce the output on the receiving flash. So if I have the receiving flash set at 1/1 power on the unit, and then I lower power to its group (a,b or c) on the transmitter to say 1/4, it shoots at 1/4. But if I set the unit at 1/4 and reduce its group to 1/8 on the transmitter, the power emitted is 1/32.

So a single unit lets you knock 7 stops off of full power (1/128 is the lowest power setting). But if you fire the flash set at 1/128 on the unit itself, and the transmitter is also set to 1/128, you get a flash of 1/16,384 power, which is (I think) a 14 stop reduction. Or more practical, you could have one on 1/2 and the other on 1/128 for a more usable 1/256 power.

Correct?

 

[Loswr]

I don't think so. I'd say the reductions would be cumulative only down to the minimum power on the flash itself (1/128th). The xenon flash tubes only have one power level - on. When you vary flash output, you're not changing the flash intensity, you're changing the duration. I doubt the transmitter can make the flash circuitry operate faster than its design parameters (for comparison, a 580/600 flash at 1/128 power has a duration of faster than 1/30,000 s).

But you have the setup, right? Just point one flash set to 1/128 at a white wall in a dark room, and dial down power on the transmitter without changing flash settings to see if it gets dimmer (or bump the ISO up one stop for each stop of power drop on the transmitter, and see if it remains constant).