Flash Guide Number Question - DPChallenge Forums

Canon EF 24-70mm f/2.8L USM

05/30/2004 02:11:12 AM · #1

Greetings...

I am seeking out some film gurus who have worked with manual flash units in the past to help me learn something....

I am flash shopping again and I'm also trying to get a better grip on 'guide numbers'. I understand the basic concept.

The Canon 550EX Speed Light is rated at 180'/55m @ 105mm @ ISO 100. Since the 550EX can adjust its own aperture between 24mm and 105mm, my questions is how is the guide number calculated for smaller flash apertures? I know the guide number comes down as the flash aperture size increases, but by how much?

doctornick

Apple iPhone 13 Pro

05/30/2004 02:22:16 AM · #2

John you might want to check this //photonotes.org/articles/eos-flash/index2.html#guidenumber

Hope it answers your question.

05/30/2004 02:26:24 AM · #3

Originally posted by doctornick:

John you might want to check this //photonotes.org/articles/eos-flash/index2.html#guidenumber

Hope it answers your question.

I have actually read that entire guide :) It touches on the subject of zooming flashes and gives some guide numbers at shorter focal lengths, but what I'm looking for is how those numbers are derived. What is the relationship between the flash aperture setting and the guide number?

Adrian Tung

Canon EOS-10D

05/30/2004 09:05:04 AM · #4

I don't think there's any formula, you have to trust the manufacturer's GN ratings on this.

The way I see it, the flash can only obtain its full GN rating when it fires all its light concentrated onto a smaller area - i.e. zoomed in to the max (105mm for the 550EX). When shooting wide, the light has to be spread out more, so the maximum GN is reduced. The way this spread occurs may be different for the various flashes, depending on the internal mechanics and reflectivity of the materials within the flash head itself.

Hope this helps,
:)atwl

Canon EF 70-200mm f/2.8L IS USM

05/30/2004 10:55:43 AM · #5

I think Adrian has hit it on the head. There isn't a mathemetical derivaton for this, because it is dependent on the specific optical characteristics of that flash unit.
The manufactrers are prolly doing actual measurements of light output to "derive" the GN vs. FoV curves.

Mick

Canon EOS-5D

05/30/2004 12:17:10 PM · #6

Originally posted by jmsetzler:

Originally posted by doctornick:
John you might want to check this //photonotes.org/articles/eos-flash/index2.html#guidenumber

Hope it answers your question.

I have actually read that entire guide :) It touches on the subject of zooming flashes and gives some guide numbers at shorter focal lengths, but what I'm looking for is how those numbers are derived. What is the relationship between the flash aperture setting and the guide number?

John, this is from the "Guide number" section of that document. It sounds to me like it's a judgement call.

"Finally, a fair bit of subjectivity goes into determining the guide number, which is presumably why it�s called a �guide.� After all, how is an �adequately exposed� subject determined? Guide values are, therefore, not a very reliable way to compare flash units built by different manufacturers. Particularly since manufacturers tend to be wildly and cheerfully optimistic when it comes to assigning guide numbers to their products."

--Mick

Canon EF 70-200mm f/2.8L IS USM

05/30/2004 12:27:29 PM · #7

Originally posted by micknewton:

Right... I understand that...

I'm just curious if there is a way to determine a revised guide number based on the zoom setting for the flash.

Mick

Canon EOS-5D

05/30/2004 12:52:40 PM · #8

Originally posted by jmsetzler:

I'm just curious if there is a way to determine a revised guide number based on the zoom setting for the flash.

In the manual that came with my 550EX (pg 121) there is a table that lists the normal (full) flash guide number for each zoom setting. The guide numbers are based on ISO 100, and are given in meters. They range from 15 at 17mm to 55 at 105mm.

I'm sorry, but exactly how they derive these numbers is beyond me. I am still learning this stuff. :)

--Mick

Message edited by author 2004-05-30 12:53:24.

Canon EF 24-70mm f/2.8L USM

05/30/2004 01:50:51 PM · #9

Originally posted by micknewton:

Originally posted by jmsetzler:
I'm just curious if there is a way to determine a revised guide number based on the zoom setting for the flash.

In the manual that came with my 550EX (pg 121) there is a table that lists the normal (full) flash guide number for each zoom setting. The guide numbers are based on ISO 100, and are given in meters. They range from 15 at 17mm to 55 at 105mm.

I'm sorry, but exactly how they derive these numbers is beyond me. I am still learning this stuff. :)

--Mick

Can you give me those numbers? I may be able to figure out how they got them... I'm looking for guide numbers on the 550ex at 24,28,35,50,and 70mm.

Thanks :)

doctornick

Apple iPhone 13 Pro

05/30/2004 01:55:59 PM · #10

17mm = GN 15
24mm = GN 28
28mm = GN 30
35mm = GN 36
50mm = GN 42
70mm = GN 46
80mm = GN 50

05/30/2004 02:45:04 PM · #11

John,
I've been interested in this as well, since I'm considering buying a 550EX. I did a back-of-the-envelope calculation, came up with the following:

Assumptions:
- The reflector efficiency in the 550EX does not vary significantly over it's "zoom range"
- The illumination power level at the plane of focus remains constant, therefore the area illuminated must be constant for a constant input energy.

With these assumptions, we can readily show that the guide number, g, varies according to the following equation:

g = x/tan(a/2)

where a is the Horz. FOV in degrees for the focal length in question. Substituting the known g = 180 @ 105mm (12.3389° HFOV) we get:

g = 19.46ft/tan(a/2)

Based on your desired focal lengths (calculated in excel):

@24mm: HFOV = 50.6205 deg, g = 41ft
@28mm: HFOV = 44.1310 deg, g = 48ft
@35mm: HFOV = 35.9343 deg, g = 60ft
@50mm: HFOV = 25.5788 deg, g = 86ft
@70mm: HFOV = 18.4199 deg, g = 120ft
@105mm: HFOV = 12.3389 deg, g = 180ft (check on calculation)

I think the above should be a pretty decent first approximation, unless the reflector efficiency does vary quite a bit.

Message edited by author 2004-05-30 14:46:01.

05/30/2004 03:50:26 PM · #12

Originally posted by kirbic:

Based on your desired focal lengths (calculated in excel):

@24mm: HFOV = 50.6205 deg, g = 41ft
@28mm: HFOV = 44.1310 deg, g = 48ft
@35mm: HFOV = 35.9343 deg, g = 60ft
@50mm: HFOV = 25.5788 deg, g = 86ft
@70mm: HFOV = 18.4199 deg, g = 120ft
@105mm: HFOV = 12.3389 deg, g = 180ft (check on calculation)

I think the above should be a pretty decent first approximation, unless the reflector efficiency does vary quite a bit.

I understand this calculation, but I can't find a relationship between these calculated guide numbers and the ones doctornick posted from the 550ex manual. The calculations don't correspond closely at all so maybe there is no relationship that can be calculated.

The lens field of view and the flash aperture setting are probably not equivalent either...

Message edited by author 2004-05-30 15:51:10.

sn4psh07

Canon EOS-10D

05/30/2004 06:19:34 PM · #13

Canon GN are optimal...

To work out real GN set flash at full power a known distance from a subject at iso 100, then vary the aperture on your lens till you get proper exposure...

GN = Dist. * aperture.

eg. GN (My 550EX) = 2.8 (meters) * (F)16 = 44.8 @ 105.

05/30/2004 08:34:11 PM · #14

Originally posted by sn4psh07:

It also depends on the reflectivity of what you use as a "target". You could use an 18% gray target, but your GN still depends on how diffuse the reflection from the target is.

05/30/2004 08:42:15 PM · #15

@John:
I looked at this a little further. As you can see, my calculations, based on admittedly simple assumptions, result in g being a linear function of FoV angle. the Canon numbers are anything but linear, and they do not fit any common, simple form of equation.
My conclusion is that :

- The flash tube is more powerful than g @ 105mm would suggest
- There is a very significant fall-off in efficiency of the reflector system, especially above 50mm.