Still don’t understand how artists pulled this off back in the day. Did she pose there the whole day? Even then the sun would move.
So many details it looks like a photo.
a sketch and then memories, is my bet while being in that room to remind you of the light
or could be off a photograph, or photographic memory.
Also possible
Far back in the olden days it would be harder, but it’s 1957, so a photo would be very possible. Especially with such a spontaneous pose. Maybe in combination with several re-sittings or just having her around, to get the skin tones just right.
As other have mentioned, they could have sketched the light and shade, then painted over… even so at midday the light might not move so fast. You can do the basic areas if colour then fine details after you know what the flesh tones are and where highlights and shadows sit. You can paint fast :)
