People really ought to have learned not to believe published guide numbers by now, and buy themselves a flashmeter. I have more flashmeters than I need, and every one of them gives the lie to published GNs - on every make and model of speedlight I've ever come across. The maker's GN is at least one whole stop optimistic. And this is verified time after time by actual exposures. The SB-25 has a real GN @ 100 ISO and 50mm zoom setting of 28 (metres), as has every other of Nikon's top line speedlights up to the SB-910. Their second tier speedlights (e.g. SB-600) are a stop less powerful, with a GN of around 20. The same goes for Canon and most 3rd party speedlights as well. Also, the zoom setting makes almost no difference to the GN beyond about 50mm. Maker's figures quoted for 105 and 180mm settings are just ludicrously exaggerated. In film days, it could be thought that short exposure reciprocity-failure was to blame for such discrepancies. Now, digital has left no doubt that makers' guide numbers are just outright lies!