>>4266251>But how did >>4266177 picture get made originally using only methods avaliable to them?It was shot on three separate black and white glass plates.
>Did they just align all three exposures?Yes, by projecting them from three separate projectors, each with R, G, and B filtered light respectively.You could not "just" look at them with naked eye.
>Could you not just expose the same frame 3 times under each filter?No, because this technique predated any actual practical color process by about 50 years. Although Autochrome process was just invented around the time Prokudin-Gorskii shot his most well known works, he decided not to use it due to cost and other factors (the Lumieres actually allowed him to beta test it before public release). But even Autochrome still worked on a principle of B&W plate and filtered light. The modern style color chromogenic emulsions (with color couplers) were not invented for about another 90 years after the three-plate technique.
It was basically an ingenious, massive hack. Also his photos could never be displayed as clearly and vividly as today with the projection equipment used back then. And he never got to see them like this during his life. Only now that the plates were digitally scanned, aligned and adjusted we can see the original scenes with brilliance that matches modern photography.
Basically he created something with a potential that was not fully realized for a 100 years. This is why I fucking love analog photography.
You can download individual uncompressed high res plates and dick around making your own composites by the way:
https://www.loc.gov/pictures/search/?sp=2&co=prok&st=grid 