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Branding is just that, branding
Some G lenses like the 20mm f1.8 are as good as a G master optically.
Some GMs are underwhelming, like the first gen 85mm.
Also see canon L lenses. A few modern RF Ls are really terrible and get mogged by sony/nikon. Some nikon S lines like the 35mm are quite shit. Samyang made a third party RF UWA prime that would have received an L designation if it were a canon product and promptly got sued, lol.
The branding doesn't tell you anything but that they didn't cut corners and included weather sealing, extra controls, and fluorine coatings and shit. It is 100% possible to beat a company's highest branded product. Just look at all of leica. The vast majority of their lenses are terrible, but cost $4000.
