300mm or maybe not.

[ebiggs1]

A futher little test shows the Sigma 120-300mm f2.8 has serious focus breathing issues.  This is from 25 ft.

The Canon EF 300mm f/4L IS USM Lens first.

Now the Sigma 120-300mm f2.8 at 300mm at the same distance.

You can try to decide whether the filters were on or off.  And, whether the IS/OS is on or off !  Tought isn't it.  Is it different than you thought or believed?

[TCampbell]

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).

[ebiggs1]

"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!