Guide number on flash guns

I know that for e.g. that the Speedlite 580EX-II has a guide number of 58 feet. But is that referring to manual mode or both?

It's actually 58 metres or there abouts

Measured at full power in manual at the widest angle setting.

yogestee wrote in post #12488527
It's actually 58 metres or there abouts

Measured at full power in manual at the widest angle setting.

Not quite.
It's actually at the tightest setting (105mm)

The guide number refers to the maximum flash output available from the unit, at full power.
The number of 58 is in metres, basically at 100iso, and f1 you will get a correctly exposed subject at 58 metres.

Gn/aperture = working distance for correct exposure at full power (at 105mm
There is a guide number/flash zoom coverage chart in the back of the manual

Echo63 wrote in post #12488563
Not quite.
It's actually at the tightest setting (105mm)

The guide number refers to the maximum flash output available from the unit, at full power.
The number of 58 is in metres, basically at 100iso, and f1 you will get a correctly exposed subject at 58 metres.

Gn/aperture = working distance for correct exposure at full power

Woops,, I knew that What was I thinking??

The other dirty little secret that we know is that FEW speedlight manufacturers list a TRUE number for the Guide Number...if you measure the output with a flash meter of known accuracy (regardless of brand), the output is overstated by about 1EV on average.

For example, my own Metz 54MZ should have a GN54 at 105mm, but it actually measures with a flash meter as GN30...what should have metered as f/18 at 3m measured f/10.
CurtisN found exactly the same type of optimism with his Sigma... https://photography-on-the.net …php?p=4029541&p​ostcount=1

And PaceAce has results for two Canon flashes and one Sunpak
https://photography-on-the.net …php?p=4031493&p​ostcount=6

Ooops, I meant that, but if I had it on ISO100, and the manual flash set to say 1/8th power, that means an eighth of the 58 metres = 7.25 metres?

400dabuser wrote in post #12488654
Ooops, I meant that, but if I had it on ISO100, and the manual flash set to say 1/8th power, that means an eighth of the 58 metres = 7.25 metres?

No. In the power progression, 1/1, 1/2, 1/4, 1/8...each step reduces output by -1EV, so 1/8 power is -3EV from full power. Assuming a flash had GN100 at full power (25' at f/4), at 1/8 power you would have GN35 (25' at f/1.4). Or stated in terms of distance change (not f/stop change), 25' at f/4 at full power becomes 8.75' at f/4 at 1/8 power. Each step in the power reduction sequence effectively divides GN by 1.414, so (100 /1.414 / 1.414 / 1.414) = 35

Wilt wrote in post #12488775
No. In the power progression, 1/1, 1/2, 1/4, 1/8...each step reduces output by -1EV, so 1/8 power is -3EV from full power. Assuming a flash had GN100 at full power (25' at f/4), at 1/8 power you would have GN35 (25' at f/1.4). Or stated in terms of distance change (not f/stop change), 25' at f/4 at full power becomes 8.75' at f/4 at 1/8 power. Each step in the power reduction sequence effectively divides GN by 1.414, so (100 /1.414 / 1.414 / 1.414) = 35

Thank you for that

There is a chart near the back of the manual that shows GN at various manual power settings.