Third party OEM is nothing new, so long as they have the tools and talent to meet the clients specs and stick to them. Who cares what the origination address is?
In my search for vintage lenses, I remember an article (for which I can’t find the link), that the majority of early Japanese lens manufacturers were all based a stones throw from one another, and consequently, copied each other.
In the 80’s Sear’s had a few good lenses made by Tokina, and it requires some research to know which ones were gems hiding under the name. Vivitar was well known for utilizing many different OEMs for their lens designs. There were several different OEM’s for a given lens (i.e. 4 different 70-200 series 1), and their serial numbers were all telling of those of whom made them, (Tokina, Komine and Kino are regarded as the worthy versions).
It all basically boils down to a great design, supported by a vendor with solid capability. Sigma sells some very decent lenses, so it’s no surprise.
Started out doing photography at the age of 6 using an uncle's old 1940 kodak brownie box camera. At 15 years of age, I decided to buy my very own 1975 Praktica SLR camera. I now shoot with a Nikon D850. I do unpaid TFP and commercial paid work.