Origin of ISO Multiple for Guide Numbers

01/05/2009 10:52:17 PM · #1

Hi. I understand the guide number: G = d x f @ ISO 100

From what I have read, when calculating the guide for ISO 200,
G(iso 200) = G(iso 100) x 1.44

WHY is it 1.44 and not 1.414 ? I other words, what is the derivation for 1.44?

Thanks,
Paul

Nikon AF-S DX Zoom-Nikkor 18-55mm f/3.5-5.6 ED

01/06/2009 08:17:59 AM · #2

bump?

Apple iPhone 5

01/06/2009 08:28:39 AM · #3

There are some complex-looking formula here which might help; //en.wikipedia.org/wiki/Film_speed

Bear_Music

Samsung Galaxy S24 Ultra

Sony 24-70mm F4 Vario-Tessar T* FE OSS

01/06/2009 08:36:56 AM · #4

I believe 1.41 is accurate; the number I always used was 1.4 (ease of mental calculation) and a quick check of online sources shows me 1.41 as the accurate number. I have yet to see a reference to 1.44; where did you get that?

R.

ETA: The number should, of course, be the square root of 2, and that's 1.41xx for sure...

Message edited by author 2009-01-06 08:39:12.

Sony 24-70mm F4 Vario-Tessar T* FE OSS

01/06/2009 11:06:14 AM · #5

That is exactly what I thought. I'm reading a book on Exposure and it was discussing the guide numbers for flash. When I saw the 1.44, I was a little curious. I have seen some web sources stating 1.44 also....very puzzling.

Only one way to find out......time to bring out a white paper and calibrate!

Originally posted by Bear_Music:

Message edited by author 2009-01-06 11:06:46.

Bear_Music

Samsung Galaxy S24 Ultra

01/06/2009 11:12:33 AM · #6

Originally posted by PGerst:

Well. 1.44 cannot be correct, because the square root of 2 is 1.41. However, if you go to a third decimal place, you get 1.414, so I'd be willing to be these various sites are propagating a bit of misinformation derived from a sloppy transcription of the number? But, I mean, c'mon... It HAS to be the square root of 2, it's all about the inverse square law...

R.

Apple iPhone 5

Nikon AF-S DX Zoom-Nikkor 18-55mm f/3.5-5.6 ED

01/06/2009 11:19:20 AM · #7

Originally posted by Bear_Music:

And we all know 'ye cannae change the laws of physics, cap'n!'

Canon RF 24-70mm f/2.8 L IS

01/06/2009 11:46:02 AM · #8

My thoughts too. Especially since at iso 400, the 1.44 is doubled, which makes me even further think 1.44 is very very wrong. Especially since the stop difference changes by 1.414

Originally posted by Bear_Music:

kirbic

Canon EOS R5

01/06/2009 08:36:28 PM · #9

Originally posted by PGerst:

My thoughts too. Especially since at iso 400, the 1.44 is doubled, which makes me even further think 1.44 is very very wrong. Especially since the stop difference changes by 1.414

Originally posted by Bear_Music:

Well. 1.44 cannot be correct, because the square root of 2 is 1.41. However, if you go to a third decimal place, you get 1.414, so I'd be willing to be these various sites are propagating a bit of misinformation derived from a sloppy transcription of the number? But, I mean, c'mon... It HAS to be the square root of 2, it's all about the inverse square law...

R.

You are most certainly correct. It is, as others have posted, 1.414, or more precisely sqrt(2). Think of a "pyramid of light," with the flash as the apex, and the base as the area illuminated. If the height is the guide number distance at ISO 100, in order to get the height (guide number) for ISO 200 we need to imagine a taller pyramid where the base is twice the area of the first (half the light per unit area, or one stop less light). That height is 1.414 times the height of the "ISO 100" pyramid. Furthermore, this is true no matter what the coverage angle of the flash is.