The claimed Guide Number of 54m of the AF540FGZ II (and reflected in its name) is only when set at its 85mm maximum zoom. It is regrettable that makers don't state up front the GN when the zoom is set at the standard lens zoom angle (corresponding to a 50mm lens on full frame). That is what they used to do, even when the unit had an integral zoom adjustment, and that allowed realistic comparisons to be made. At least they still quote the GN for ISO 100 sensitivity, but I wonder how long they will keep doing that.
Interested, I had to dig deep to find what its standardised GN is (it's 45m, still impressive) and even what its maximum zoom setting is in full frame terms (it's 85mm). Neither of those figures is mentioned in the advert, or even in the the unit's PF review page, despite ten reviews.
PENTAX AF 540FGZ II reviews - Pentax Camera Accessory Review Database 
Originally posted by K2 to K50 
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