RG
Appreciate your scientific definitions of the sweet spot but in general your definition of myths are simple "in general" situations to what most have found to be good starting points for "various" lenses on the sweet spot.
It's true about Leica, but myth or not, it's common for good starting points for the popular "in general" lenses to be about two stops. Doesn't mean exactly but there is no need to rack our brains to get so specific with the sweet spot.
My own bench tests from a visual stand point on everything from the past 21 years and every lens used from Zeis to Mamiya to Tamron with Canon and Nikkor in between has hit the sweet spot between 2 and 2.5 stops from the widest aperture almost every single time, regardless of manufacturer's lenses bench tests. This is why you hear it constantly repeated in lamen terms and it's why I prefer to generalize the sweet spot in most cases.
It's always best with the individual and his or her camera to do their own field test regardless of what companies say their lenses will do. Sweet spots will vary per individual tastes, cameras, film or digital, 1.5x factor or full frame sensors, digital noise, mirror alignment, etc, for their view points.
Most importantly a photographer should not be limited to the sweet spot and can very easily limit him/her to constantly thinking they have to be shooting at the "sweet spot".
For instance the original poster has never been able to tell a difference and now he will probably be limiting himself trying to find a sweet spot thinking he will become a better photographer for it. ;-)
J T
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