How to extent guide numbers to different film speeds, for manual full power flash?

[Helge]

I've seen some conflicting advice on how to uprate guide numbers. Like the GN at 1m @ ISO 100 is 36.
The advice online is scant for some reason, but two conflicting methods come up.
Say I want the GN for 1600.
One says to just multiply by the number of stops. Like: four stops from 100 to 1600. So 36x4 = 144
The other says to multiply by 1.4 for each stop: So 36x1.4 = 50.4x1.4 = 70.56x1.4 = 98,78x1.4 = 138,29 @ 1600

Pretty close though. The last one seems to agree with the short table on the back of a packet of bulbs.
But one of them gotta be right though.
Which one?

For bounced flash it doesn't matter a whole heck of a lot of course, a lot is up to slack and experience. But it's still nice to be able to be precise.

 

[AgX]

The factor for an aperture stop but also for a rise in guidenumber per doubling of film sensitivity is the square root of 2

 

[Helge]

The factor for an aperture stop but also for a rise in guidenumber per doubling of film sensitivity is the square root of 2

That was fast. Thanks! Of course it’s SQR2. Logical.

 

[MattKing]

One says to just multiply by the number of stops. Like: four stops from 100 to 1600. So 36x4 = 144
The other says to multiply by 1.44 for each stop: So 36x1.44 = 50.4x1.4 = 70.56x1.4 = 98,78x1.4 = 138,29 @ 1600

The second version is only different because the square root of 2 isn't 1.44, but rather 1.4142

 

[Nodda Duma]

Helge I generated Guide Number curves in an Excel spreadsheet to do exactly what you want, based off data on the back of various flashbulbs. If you can use Excel I’ll upload the file

 

[Chan Tran]

Instead of multiply by the square root of 2 for every stop you can do this.

New GN= GN@100 x √( New ISO / 100)

 

[wiltw]

NewI've seen some conflicting advice on how to uprate guide numbers. Like the GN at 1m @ ISO 100 is 36.
The advice online is scant for some reason, but two conflicting methods come up.
Say I want the GN for 1600.
One says to just multiply by the number of stops. Like: four stops from 100 to 1600. So 36x4 = 144
The other says to multiply by 1.4 for each stop: So 36x1.4 = 50.4x1.4 = 70.56x1.4 = 98,78x1.4 = 138,29 @ 1600

Pretty close though. The last one seems to agree with the short table on the back of a packet of bulbs.
But one of them gotta be right though.
Which one?

None of the above. Multiplier
For ISO 100 film GN, +1EV guide number is 1.414X, +2EV is 2X, +4EV is 4X

• ISO 100, assuming starting GN 100,
• ISO 200 = GN ~140,
• ISO 400 = GN 200,
• ISO 1600 = GN 400

Using above GN, assuming 25' distance from flash to subject

• ISO 100 = f/4
• ISO 200 = f/5.6
• ISO 400 = f/8
• ISO 1600 = f/16

 

[Chan Tran]

None of the above. Multiplier
For ISO 100 film GN, +1EV guide number is 1.414X, +2EV is 2X, +4EV is 4X

• ISO 100, assuming starting GN 100,
• ISO 200 = GN ~140,
• ISO 400 = GN 200,
• ISO 1600 = GN 400

Using above GN, assuming 25' distance from flash to subject

• ISO 100 = f/4
• ISO 200 = f/5.6
• ISO 400 = f/8
• ISO 1600 = f/16

What's about ISO 125, 160, 250, 320? etc..

 

[swchris]

None of the above.

Why "none of the above"? Taking the times 1.4 as containing rounding errors, the numbers are the same.

 

[wiltw]

Why "none of the above"? Taking the times 1.4 as containing rounding errors, the numbers are the same.

...if you do not care about exposure accuracy, 'about the same' does not work when you stack the multiplier (just as the OP did). f/16 vs f/14.75... resulting in overexposure, is that 'close enough' for shooting transparencies?...not for many folks.