John,
I'm not arguing semantics. The simple fact of life is that GN are stated for ISO 100. It is 'normal' to do so.
Without such a norm, these thingies make no sense at all.
It's bad enough that there are two systems in use: metric and imperial.
It's even worse that manufacturers began ignoring that other norm, that 'dictated' that the GN should be stated for a normal reflector, i.e. covering the field a 'standard' lens sees.
Metz, for instance, (like Hanimex mentioned earlier in the thread) use the GN (yes, at ISO 100) in the type designation of their units. It was instantly apparant that a Metz CT 45 unit put out one stop more light than a Metz CT 32 unit. Makes (or rather: made) life very easy!
Now they abandoned that very usefull 'normed' scheme to increase the marketing clout. A Metz 58 unit sound impressive, almost on par with a Metz 60 unit. But in fact it puts out less light than a Metz 45 unit. The trickery, the attempt to fool the customers into believing it is a really powerfull unit, is in stating the GN for the tele-setting of the reflector, i.e. with the light concentrated on a smaller field.
The Metz 76 is even worse: despite the even more impressive GN, it too puts out less power than the Metz 60 units.
A cheap trick, that only works (and mark this) because it makes use of the fact that people think the GN-specification adheres to the 'norm'.
What it shows, is that a 'guide' stops being usefull as a guide as soon as you start fiddling with it.
And it would be terrible if you had to read all the small print to find out for what ISO the GN stated is true.
Luckily, ISO 100 still is the norm.
It's just like the lower light level limit stated for light meters. It too is given for ISO 100.
Just like with GNs, to make such a specification instantly usefull.
That's just the way it is. And it's not semantics. It's terribly usefull.
Now, you said that GNs change with ISO.
The best reply to that is: "Duh!"
And what's your answer to the question about the Hanimex's GN?
And finally: yes, as with all 'norms' you have to know how the numbers are produced.
Or rather, you have to know that a number given can be trusted, and is usefull.
I already wrote a bit about conditions, so i think it would suffice to limit my further answer to "Duh!" again.
