Converting old GNs

Pippy_Neville

Pippy_Neville

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Am I right in thinking that if I have a really old flash that is listed in its manual as GN22 at DIN18 then it would be GN16 in normal terms?
My reasoning is: DIN18 = ISO50 which (I think) is a full stop faster than ISO100. And f16 is a full stop faster than f22. Ergo GN22 adjusted by the same ratio for the ISO = GN16.

So did I get it about right, and did I get it right for the right reasons!

Thinking about using this as a rimlight if I can get it to trigger as it's non adjustable, I'm guessing it'll be suitable?

Cheers,
pip
 
Pippy_Neville said:
Am I right in thinking that if I have a really old flash that is listed in its manual as GN22 at DIN18 then it would be GN16 in normal terms?
My reasoning is: DIN18 = ISO50 which (I think) is a full stop faster than ISO100. And f16 is a full stop faster than f22. Ergo GN22 adjusted by the same ratio for the ISO = GN16.

So did I get it about right, and did I get it right for the right reasons!

Thinking about using this as a rimlight if I can get it to trigger as it's non adjustable, I'm guessing it'll be suitable?

Cheers,
pip
Click to expand...
ISO 50 is SLOWER then ISO100 not faster
 
Chaz Photos said:
ISO 50 is SLOWER then ISO100 not faster
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D'oh! Yes you are right of course Chas, I've used the wrong term by 180 degrees! What I should have said of course is something along the lines of:
ISO50 is 1 stop slower than ISO100 so to compensate you make aperture 1 stop faster ie from f22 to f16; so to adjust GN for different ISO's you compensate by same ratio as aperture change... which in this case is a handy change from GN22 to GN16.
I'm confusing myself now!
 
Yes, DIN doubles its value by every increment of 3, therefore

18 = 50
21 = 100
24 = 200
27 = 400
etc

So, f/11 @ DIN 18 = F/16 @ 100 iso
 
Garry Edwards said:
Yes, DIN doubles its value by every increment of 3, therefore

18 = 50
21 = 100
24 = 200
27 = 400
etc

So, f/11 @ DIN 18 = F/16 @ 100 iso
Click to expand...


Aah, so I got one bit right then!
But my original question remains - is GN22 at DIN 18 same as GN16 at ISO100?
Come on you old school toggers, this is probably easy for you!
 
Work it out for yourself...
Make it even easier by using the same expression of sensitivity, i.e. 50 ISO/100 ISO or 18 DIN/21 DIN

Now divide distance, say 1 (metre), into guide No.
22/1 = f/22
16/1 = f/16
You can see that there is exactly 1 stop difference, therefore GN22 @ 18 DIN is identical to GN16 @ 21 DIN
 
Pips

Guide number = distance * f stop ( at a particular ISA). It can be expressed in either feet or mts.

So, in your example, lets say its GN 22 at 18 DIN ( or 50 ISA).Also ,lets say its in mts( you got to check the flash spec for that)

Therefore : if you set the f to 22. Then 22/22 = 1 mts. So you can fully illuminate the subject at 1 mts with 22 f and a 50 ISO film. With a 100 ISO film, you have to use a f stop of 32 for the subject to be 1 mts ( to compensate for a higher sensitive film) .

So the equivalent GN will be : 32*1 = 32 (mtrs)


Ujjwal

Edit : Garry : I see that you have arrived at a different result; where am I going wrong? 100 ISA film will need a smaller aperture opening ( f32 ) compared to 50 ISA film ( f22); other things remaining same ; isn't it?
 
ujjwaldey8165 said:
Pips

Guide number = distance * f stop ( at a particular ISA). It can be expressed in either feet or mts.

So, in your example, lets say its GN 22 at 18 DIN ( or 50 ISA).Also ,lets say its in mts( you got to check the flash spec for that)

Therefore : if you set the f to 22. Then 22/22 = 1 mts. So you can fully illuminate the subject at 1 mts with 22 f and a 50 ISO film. With a 100 ISO film, you have to use a f stop of 32 for the subject to be 1 mts ( to compensate for a higher sensitive film) .

So the equivalent GN will be : 32*1 = 32 (mtrs)

I got a different answer to you because I answered the question "is GN22 at DIN 18 same as GN16 at ISO100?". The answer is that it is the same.
What you did was to explain how guide numbers are calculated. You're not wrong because you have correctly explained how guide numbers are calculated, although in practice it's normally only a manufacturer (or tester) who works that way round, people normally know the guide number and use my calculation method instead, to work out the f stop.
What you did was to give the correct answer to a different question - your answer was the right one to what might be the next question - "which lens aperture should I use at a distance of 1 metre at both 50 ISO & 100 ISO?"


Ujjwal

Edit : Garry : I see that you have arrived at a different result; where am I going wrong? 100 ISA film will need a smaller aperture opening ( f32 ) compared to 50 ISA film ( f22); other things remaining same ; isn't it?
Click to expand...
 
Oops, quote went a bit wrong there:'(
 
Ah, got it

Since GN are usually quoted at 100 ISA ( at least the ones I have seen); I was trying to convert his example to a 100 ISA equivalent.

From there, its easy to calculate the F stop or the distance at any ISA.
 
I'm confused :thinking:

GN22 at 18DIN is GN31 at ISO100. The difference is one stop, so just multiply the GN by 1.4x.

Isn't that it? Or have I missed the question?
 
Blimey, it seems that this may not be as simple as I at first thought.

Perhaps the only way is to actually measure it with a flash meter at various distances, it's possible there is not an easy mathematical ratio unless you bring in the inverse square law - which may be just a little bit over the top for what I was trying to work out!
 
HoppyUK said:
I'm confused :thinking:

GN22 at 18DIN is GN31 at ISO100. The difference is one stop, so just multiply the GN by 1.4x.

Isn't that it? Or have I missed the question?
Click to expand...

That's it :)
 
Pippy_Neville said:
Blimey, it seems that this may not be as simple as I at first thought.
Click to expand...

Not really ;) If you think you have a difficult question, you go looking for a complicated answer.

Actually, Ujjwal got it right first.

You were also on the right track Pips, but got it the wrong way round, then having put that right you went and reversed it again.

If you start by saying higher ISO/DIN produces a higher guide number, then you'll move in the right direction.

It's a stop difference, must be higher, therefore x1.4. That's 31 or 32 with roundings. But don't go thinking that guide numbers have too much to do with light output - they are often expressed with different parameters and I have yet to see any kind of recognised standard. Just be sure you are comparing like with like, at least the best you can. The big cheat these days is to quote ISO100/metres so it looks legit, but then don't mention the fact that it's only with the flash zoom head on max, like 105mm, which makes a huge difference.

FWIW, the GN is often quoted in the model spec, eg Canon 580EX has a GN of 58. Unless you're Nikon :lol:
 
HoppyUK said:
Not really ;)

If you start by saying higher ISO/DIN produces a higher guide number, then you'll move in the right direction.

It's a stop difference, must be higher, therefore x1.4. That's 31 or 32 with roundings. But don't go thinking that guide numbers have too much to do with light output - they are often expressed with different parameters and I have yet to see any kind of recognised standard. Just be sure you are comparing like with like, at least the best you can. The big cheat these days is to quote ISO100/metres so it looks legit, but then don't mention the fact that it's only with the flash zoom head on max, like 105mm, which makes a huge difference.

FWIW, the GN is often quoted in the model spec, eg Canon 580EX has a GN of 58. Unless you're Nikon :lol:
Click to expand...

:thumbs:
The fact of the matter is that guide numbers is all we've got, short of actually measuring the light with a meter, which is the only reliable way of calculating the real world output.
To some extent, manufacturers can be responsible for misleading figures, for example some may produce figures that aren't replicated under real world conditions - testing a hotshoe flash at the long end of the zoom range is a good example, testing either hotshoe flash or studio flash in a small room painted white is another one. But as you say, there is no benchmark testing process and a guide number measured at the top of Mt Everest would be just as unhelpful as one produced in a small white room.
My own testing procedure, FWIW, is to test with the standard reflector in a studio much larger than any home studio but where there is still some contribution from reflective surfaces. This method does at least produce a figure that can be matched or bettered under any normal use.
 
Pippy_Neville said:
Perhaps the only way is to actually measure it with a flash meter at various distances
Click to expand...

No. You should be able to work it out mathematically.

Guide numbers are (were) given with flashes so that the exposure could be worked out without a meter.


Steve.
 
If its guide number is 16 at ISO 50 then at ISO 100 it will be 16 x the square root of 2 i.e. 22.6

Therefore, if you want to use this flash at ISO 100, divide the guide number by the distance (I think this one is in metres).

So for a subject 3 metres away, 22.6/3 = 7.53, nearest aperture = f8


Steve.
 
Steve Smith said:
No. You should be able to work it out mathematically.

Guide numbers are (were) given with flashes so that the exposure could be worked out without a meter.


Steve.
Click to expand...

Not really. The mathmatical calculation, i.e 1.414 will only work if the guide number is accurate in the first place - which as already explained depends on the testing conditions, which is very unlikely to match any given shooting conditions
 
Garry Edwards said:
Not really. The mathmatical calculation, i.e 1.414 will only work if the guide number is accurate in the first place - which as already explained depends on the testing conditions, which is very unlikely to match any given shooting conditions
Click to expand...

Yes but we shouldn't be cynical and assume the guide number is wrong!

The guide number was given for one reason only and that was to work out exposure without using a meter. With negative film it should have been accurate enough.

If the guide number was not right then the manufacturer has failed.

I do know what you mean though. When the Vivitar 283 and 285 flashes were introduced, my father did some testing and discovered that of all the similar sized flashes available at the time, only the Vivitars were as powerful as their guide numbers indicated (which contradicts my opening sentence!).


Steve.
 
Steve Smith said:
Yes but we shouldn't be cynical and assume the guide number is wrong!

The guide number was given for one reason only and that was to work out exposure without using a meter. With negative film it should have been accurate enough.

If the guide number was not right then the manufacturer has failed.

I do know what you mean though. When the Vivitar 283 and 285 flashes were introduced, my father did some testing and discovered that of all the similar sized flashes available at the time, only the Vivitars were as powerful as their guide numbers indicated (which contradicts my opening sentence!).


Steve.
Click to expand...
I think that we do need to be cynical. The statement that The guide number was given for one reason only and that was to work out exposure without using a meter may not be correct, another possible reason is that the guide number may persuade people to buy one particular product instead of another.

And anyway, as already explained, even if the guide number is correct in one particular shooting situation it can be very different in another one. The ONLY way to get accurate exposure is to measure it
 
Garry Edwards said:
I think that we do need to be cynical. The statement that The guide number was given for one reason only and that was to work out exposure without using a meter may not be correct, another possible reason is that the guide number may persuade people to buy one particular product instead of another.
Click to expand...

Possibly.

I suppose it's a bit like the manufacturers of car audio equipment who decide to use different standards such as Peak Music Power Output instead of the industry standard RMS to give their products inflated power outputs which make them appear more desirable (to some people).

In the case of the flash though, I don't think that was the manufacturers' intention even if it was the result.

Garry Edwards said:
And anyway, as already explained, even if the guide number is correct in one particular shooting situation it can be very different in another one. The ONLY way to get accurate exposure is to measure it
Click to expand...

I am going to stick to my original statement on this. Most flashguns are sold to amateurs who do not have and probably never will buy a flash meter. Therefore, the guide number was issued as a guide to exposure.


Steve.
 
I am with Steve on this one.

1.Guide Number is just that - a guide. Its no substitute for metering.

2. Its useful to amateurs to get an approximation of flash setting - and for some folks as a rough and ready measure of comparison of strength of flash. So its better than having nothing.
3. There is really no reason to believe that it was invented to deceive customers. That spin doctoring took over at a later stage is another matter.

4. Given the latitude of films; and the inaccuracy of metering etc; its good enough for most photographers who used it

5. Its no different than the mpg figures - they are 'as measured' under certain test conditions. Yet mpg figures are widely used in various places, including for taxation

6. Yesterday, I measured 3 of my flashes in various different rooms; at 2.5 mts distance as specified by the flash manufacturer. All of the results were accurate within limits of error ( max 1 stop; which is something films can easily tolerate). Incidentally, the factor of 1.414 will always work for a certain given condition; it will simply be in error by a defined f stop.

Ujjwal
 
AFAIK guide numbers were 'invented' for use with flash bulbs. There were no more than 2 types in popular use, so the guide numbers were very useful because they indicated which bulbs to use for a particular application - either the expensive ones or the very expensive ones:)
Of course, they were no more than a guide because different flashguns had very different reflectors. People had to make allowances for this when interpreting the guide numbers. Also, there were no flash meters available at that time.

But the situation is different now. The guide number has become a marketing tool and it CAN be accurate because the manufacturers have control both of the actual flash output and the equipment that accomodates it, if only the manufacturers test their equipment under realistic conditions, which some do and some don't. And most amateur photographers are using digital now, which has zero real lattitude for exposure errors.
6. Yesterday, I measured 3 of my flashes in various different rooms; at 2.5 mts distance as specified by the flash manufacturer. All of the results were accurate within limits of error ( max 1 stop;
Click to expand...
But 1 stop is an error of 100%!
 
Garry Edwards said:
But 1 stop is an error of 100%!
Click to expand...
Not if you take the MAX and min of the range the flash will cover IE the min amount of light for the shortest distance to the Max light to the max distance it will light this will be a large number of stops
 
Chaz Photos said:
Numbers are used to confuse the buyer it’s like sound output so many put PMP and not RMS as you get a bigger number yet it’s no more sound then a RMS number that is a fraction of the PMP
Click to expand...

That's what I said earlier!


Steve.
 
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