03-10-2018 04:04 PM
A 5% rounding error is fairly typical for lenses... e.g. if a lens was marked "100mm" focal length and actually had a 96mm focal length... that would be ok.
Canon's 70-200mm lenses maintain the focal length to within 5% as you focus. That lens (at the 200mm end) turns out to be something like 190mm if you focus to the closest focusing distance. It's unique becaue the 3rd party lenses in the same category all breathe quite heavily... they're something like 130-140mm (even though the zoom ring indicates 200) when focused to closest focusing distance (so it's more like a 30-35% reduction vs. Canon's 5% reduction).
03-11-2018 12:41 PM
"A 5% rounding error is fairly typical for lenses..."
I was not trying to justify or condemn it just stating the fact.
"...becaue the 3rd party lenses in the same category all breathe quite heavily... "
Not all. Some to most, yes, but not all. To a point the Nikkor 70-200mm f2.8 also falls into that category of 'heavy breather'. Hardly a 3rd party lens!