Guide numbers -- how to calculate?

[KoNickon]

I know there is a mathematical formula to calculate a flash's guide number, but without an instruction manual for the flash that tells you the guide number (or a table attached to the flash from which you can derive it), how can you determine it? I have a number of small flash units that seem to work OK, but how to tell how strong they are without trial and error?

 

[konicaman]

Math is not my strong side, but I seem to recall that the name of the flash often contains the guide number at ISO 100 measured in meters. E.g.: Speedlite 270 EX = G.N. 27

 

[KoNickon]

That was my thought too -- but the flash I'm thinking of is a Kako 818. Not sure what I can do with that.

 

[Bill Jones]

That was my thought too -- but the flash I'm thinking of is a Kako 818. Not sure what I can do with that.

The guide number is the distance times the value of the f-stop. It can be expressed in either feet or meters. Looking at the display on an 818 I would assume that it is a GN of 15 in meters or 50 in feet. The picture I saw had f5 lined up at 100 ASA (now ISO) and the distance pointer aiming at 3m. Hence 3 X 5 = 15

Bill

 

[CMur12]

I used to know how to do this.

To the best of my memory, for a guide number in feet:
1. Set the ISO/ASA to 100
2. Look at the aperture on the scale corresponding to a distance of 10 feet
3. Multiply that aperture times 10 for the guide number.

I'm not absolutely sure about this and I no longer remember how to do it in meters. I might have just taken the guide number in feet and done a feet-to-meters conversion.

- Murray

PS. We need Chris Crawford to weigh in on this one. He would certainly know the answer.

 

[Bill Jones]

I used to know how to do this.

To the best of my memory, for a guide number in feet:
1. Set the ISO/ASA to 100
2. Look at the aperture on the scale corresponding to a distance of 10 feet
3. Multiply that aperture times 10 for the guide number.

I'm not absolutely sure about this and I no longer remember how to do it in meters. I might have just taken the guide number in feet and done a feet-to-meters conversion.

- Murray

PS. We need Chris Crawford to weigh in on this one. He would certainly know the answer.

Murray,

Your method is correct also. It could be done for any marked distance as long as you multiply the f number by the corresponding distance.
Same applies to meters. Set the iso/din to 100/21 read an aperture and distance combo and multiply them together.

eg using your text:

1. Set the ISO/ to 100/21
2. Look at the aperture on the scale corresponding to a distance of 3 meters
3. Multiply that aperture times 3 for the guide number.

Note you can use any convenient distance figure as long as it is the same in steps 2 and 3

Bill

 

[KoNickon]

Yes, f stop X distance to the subject = the guide number; I looked it up. What threw me is that the resulting number, in feet, can be rather high, especially when considering the flash is apparently not that powerful. But I would expect that the chart/calculator dial on the flash wouldn't allow for a film speed/f-stop/distance combination that exceeds the flash's capability.

Another thing that's confusing -- the guide number is talked about as if there is only one such number. Of course, it varies depending on the film speed.

 

[Bill Jones]

Yes, f stop X distance to the subject = the guide number; I looked it up. What threw me is that the resulting number, in feet, can be rather high, especially when considering the flash is apparently not that powerful. But I would expect that the chart/calculator dial on the flash wouldn't allow for a film speed/f-stop/distance combination that exceeds the flash's capability.

Another thing that's confusing -- the guide number is talked about as if there is only one such number. Of course, it varies depending on the film speed.

Nick,

The reason for only listing one GN is that they change by a factor of 1.4 ( actually square root of 2, 1.414...) instead of 2 when you change the ISO number because they are related to f-stop numbers which change by the same 1.4 (etc).

So a 100 iso GN in feet of 100 would be an iso 200 GN of 140 in feet and an iso 50 GN of 70 in feet. For each doubling of iso multiply by 1.4 (eg iso 800 is double to 200 double to 400 double to 800 so GN is 100 X 1.4 X 1.4 X 1.4 = 100 * 2.8 or GN of 280 ) For each halving of iso multiply by .7 (eg iso 25 is halve to 50 halve to 25 so GN is 100 X .7 X .7 = 100 * .5 or GN of 50 )

Because this gets confusing its easier for manufacturers to list just one GN and let you do the corrections on the dial provided on the flash.

Bill

 

[KoNickon]

Nick,

The reason for only listing one GN is that they change by a factor of 1.4 ( actually square root of 2, 1.414...) instead of 2 when you change the ISO number because they are related to f-stop numbers which change by the same 1.4 (etc).

So a 100 iso GN in feet of 100 would be an iso 200 GN of 140 in feet and an iso 50 GN of 70 in feet. For each doubling of iso multiply by 1.4 (eg iso 800 is double to 200 double to 400 double to 800 so GN is 100 X 1.4 X 1.4 X 1.4 = 100 * 2.8 or GN of 280 ) For each halving of iso multiply by .7 (eg iso 25 is halve to 50 halve to 25 so GN is 100 X .7 X .7 = 100 * .5 or GN of 50 )

Because this gets confusing its easier for manufacturers to list just one GN and let you do the corrections on the dial provided on the flash.

Bill

Right -- still, why not state "guide number of X at ISO 100"?

 

[Bill Jones]

Right -- still, why not state "guide number of X at ISO 100"?

Nick,

I need to make a correction to my post #4 above. Re-looking at the e-bay photo the 3m mark lines up more with f6 and would yield a GN of 18 in meters or 60 in feet. Hence the 18 in the flash's name.

As for why only list GNs at ASA/DIN 100/21 I don't know. My guess is some "genius" thought that would be an information overload for a typical user and if every maker marked their flashes way this (which they did) then you could use it to compare the relative power of any flash to any other one. Many flashes have the metric GN as part of their name.

Bill

 

[KoNickon]

Thanks, Bill -- I think you're right about the 18 -- an approximation at best but good enough for Kako. (So if the 18 is the GN, then what's the other 8?🙂)

 

[Bill Jones]

Thanks, Bill -- I think you're right about the 18 -- an approximation at best but good enough for Kako. (So if the 18 is the GN, then what's the other 8?🙂)

Nick,

Who knows. Probably just a marketing or engineering addition to make a unique model number.

Bill

 

[KoNickon]

Nick,

I need to make a correction to my post #4 above. Re-looking at the e-bay photo the 3m mark lines up more with f6 and would yield a GN of 18 in meters or 60 in feet. Hence the 18 in the flash's name.

As for why only list GNs at ASA/DIN 100/21 I don't know. My guess is some "genius" thought that would be an information overload for a typical user and if every maker marked their flashes way this (which they did) then you could use it to compare the relative power of any flash to any other one. Many flashes have the metric GN as part of their name.

Bill

I took a look at my 818 last night and actually the 3m mark lines up more with f5 or a little wider, as you said in your post #4. Whatever. It's an approximate value, as you said.