measuring 580ex output w/ light meter - strange results

[scottf2]

Hi all. I just got a new Sekonic 358 light meter, and have been playing around with it by measuring my various strobes (White Lightning 1600, AB 800, 580ex and Viv 285). I was performing some experiments to see how the various strobes compared to each other power-wise. I measured out a 5 foot long piece of string and then set up each strobe to fire at full power, using the string to make sure I was always at the same distance from the flash - so far, so good, and the results were about what I would have expected when comparing the various flashes at full power.

When I started playing around with the 580 power settings, things got strange. I basically found that the power settings had little or no effect on the measured output of the flash, except at the extremes. So, the difference between full power through to 1/32 was NIL... once I got down to 1/64, the output was lower by two stops, and at 1/128, lower by another stop. But between 1/1 and 1/32, no difference.

I had the flash set on Manual, manual zoom at 50 mm, and triggered it both by PW (using the Sekonic PW module) and also by "manual" flash triggering mode (set the sekonic on manual flash mode, and then fired the PW manually, taking the Sekonic transmitter out of the loop.)

I was using an external battery (turbo 2x2) and waiting for full recharge according to the indicator light on the flash. There was the expected difference in recharge time at the varying settings (ie: it took longer to recycle at 1/1 than at 1/32, etc.)

I performed the experiment multiple times with the same result. All of the other flashes worked as expected: the meter readings reflected the changes in power indicated by the flash settings.

Can anyone think of a good explanation for this other than that I have a defective flash?

The next obvious step will be to take a bunch of exposures at the same camera setting and varying flash power and see if they're exposed differently, but I didn't have time to do that... yet.

Any suggestions/explanations?

Thanks,
Scott

[PacAce]

Did you check to see if the 580eX was in Master mode? If so, set it to "Off" and try it again.

[scottf2]

Hi Leo - Thanks for the reply - It was not in master mode...

[Curtis N]

What reading are you getting at the higher power levels?

[scottf2]

I'm not home right now where I have it all written down, but if I remember correctly I was getting right around f22 at 100 iso at (approx) 5 ft. away at full power on the 580ex. The Viv 285 was showing about 1/3 stop above f16 at the same distance (so it was about 1/2 stop less powerful than the Canon)

It was set at the 50mm setting (Viv was set on "normal" - the middle setting).

Is there any way that the 580 could have been firing a pre-flash, even though it was a) off camera, triggered by PW and b) slave mode set to "off"? I doubt it, but... maybe?`

[TMR Design]

I'm not sure what you might be doing wrong or if the flash is having a problem but when I have my 580EX I did exactly what you're doing and found it to be dead accurate in terms of the power adjustment. When I moved 1/3 stop that was reflected in the reading I got.

[Curtis N]

If you started at full power and went down from there, the only thing I can think of is that somehow the meter wasn't taking a reading and you were looking at the previous reading unknowingly.

But I don't have a plausible scenario that would cause that.

[scottf2]

Thanks Curtis - As I said, it is a new meter, but I've used it a bunch already, read the manual thoroughly, and got expected results for the other flashes, so I was wondering if there was something I wasn't thinking of, aside from either user error or a malfunctioning flash. I'll do some more experimenting tonight and see what happens.

Scott