Full disclosure, this is also true of flash, but because all flashes have a ‘max’ GN stated that we believe to be equivalent, we mistakenly believe that a GN is an actual standard.
Whereas two flashes with identical power can advertise different GN’s because one only zooms to 90mm and the other zooms to 135. So you need to read the entire description of the GN, not just the headline number.
Yes, agree. With digital TTL flashes from the major camera manufacturers, they
would provide a GN for flash with
at certain narrow angle of coverage, like 200mm, and base the flash . model designation on the metric GN, And you could download a copy of the user manual and look at a
'in common' angle of coverage to compare power outut (like using 50mm FL coverage). Then, by comparing the 50mm FL coverage angle GN, you had an indicator about the marketing BS (higher GN model designation at 200mm zoom head vs. lower GN for 105mm zoom head). So I know my Metz 45 flashes have higher actual output power than the highest-end model Canon flash, even though the model designation GN seemed to be higher on the Canon.
It seems that the China-manufactured flash units that so many buy for the low price, publish NO TABLE of guide numbers at all even in owner manuals, So there is little for the consumer to use as guidance in the selection process between different vendors and models, as one has no information at all about any
'in common' coverage angle, nor any objective assessment using ISO 100 at a certain shutter speed and aperture....obfuscation, like LED source products. The consumer gets more confusion and less information, and less customer service and product support today than was offered 10 years ago...sad!