The 28-70 SL is made in Japan. Leica doesn’t have a plant in Japan. Therefore, a third party manufacturer makes this lens for them. Sigma manufacturers its lenses in Japan and it just so happens they make a lens with the identical optical design. It isn’t a huge leap in logic to suspect that Sigma makes the 28-70 SL. They are certainly capable of it. Glass would be the same, maybe stricter tolerances, coatings could be different, casing obviously different and to Leica specs.
That's possible, and doesn't matter, who makes it, for as long as it doesn't cost a kidney, and the quality is right on the money.
Just wait, and see, how it performs in real life.
IQ, solid make, light & compact, affordable,.. is what I'm after in standard zoom.
It seems, there no another zoom, like this. I'm quite exited.
Diagram of Leica lens on the left, Sigma lens on the right.
It shows different spacing, size, and curvature of elements.
We don't know, what glass was used. How, and where the glass was backed.
But, we know, the coating is of Leica. And, that alone matters a lot.
Build Q of Leica, we know, it's superior with tight sealing the whole lens.
The sealing helps to stop getting dust inside, when a zoom extends, and focuses.
All of this in light & compact solid package with price tag similar to Lumix S 24-70/2.8 Pro ... well, that sets a new standard.
If one doesn't really needs that extra 2mm at wide end ... it's no need to do heavy lifting all day long, and everyday.