No-one needs to "show" it is true because it is very widely known
across the industry.
The Nikon users who buy pro quality Nikon glass don't need to worry
because their lenses were assembled in a Nikon-owned factory.
The people who buy and use the cheap and nasty consumer-grade
Nikon-branded lenses that are made by contractors don't care. What
matters to them is the Nikon badge which sets their lens apart from
one with a Cosina, Tokina or Tamron badge. Who actually made the lens
is of no concern. It's a "Nikkor" because it says so on the lens and
that is all that matters to them.
However, there are some widely known examples of camera brand
equipment being revealed to be made by contractors. I remember long
ago calling Nikon UK's SLR Product Manager to enquire when Nikon UK
would be offering the FE10 35mm SLR. They already offered the FM10
but I wanted a replacement for my FE2, with aperture priority AE.
The man from Nikon UK told me that the FE10 would not be imported
because it wasn't of high enough quality. When I expressed surprise
that any Nikon product wasn't considered of sufficiently high quality
to sell in the UK, and pressed him for an answer, he reluctantly
admitted that it was not made by Nikon, but by a contractor.
It turns out that both the FM10 and FE10 were versions of the Cosina
CT1. The FM10 was reasonably reliable and sold well. The FE10 was
chronically unreliable so was not imported. I don't think either was
ever officially imported to the USA. There were also two "Nikkor" kit
lenses made by Cosina for these cameras, a 35-70mm f/3.5-4.8 (which
bore a close external resemblance to the discontinued AIS Nikkor
35-70mm f/3.3-4.5 but was a lousy performer) and a 70-210mm.
All this is so widely known that it appears on Wikipedia. But did
Nikon Japan ever officially admit it? If they did, it was only
because Cosina's Mr Kobayashi wanted it to be known that Cosina
manufactured camera bodies and/or lenses for Canon, Nikon, Pentax,
Olympus and Vivitar and told everyone who would listen. ;-)
It is very well known that Cosina made 100mm f/3.5 macro lenses for
Pentax and Canon plus SLR bodies for Canon, Nikon, Olympus, Konica and
Yashica over the years.
In the 1970s/80s there was constant pressure to produce ever cheaper
SLR cameras and lenses. The Nikon Series E lenses were a lower cost
range than the all-metal Nikkors but still had good performance. They
were designed by Nikon but entirely manufactured by contractors. Some
of them were so good that their optical design was later used in "full
Nikkors", notably the 28mm f/2.8, 50mm f/1.8 and 70-210mm f/4. It
isn't clear which contractors made them; Tokina and/or Cosina are the
most likely but Kino Precision may also have been involved. Several
of the design team subsequently left Nikon and joined Tokina where
they produced some outstanding designs.
At the time, every manufacturer was making its own "kit" lenses and
one particular contractor saw an opportunity to lover costs by making
one basic design in huge numbers and customising it to each
manufacturer's mount. The result was an extremely cheap 35-70mm lens.
Later, a 35-80mm was made, and this became quite controversial because
of its poor optical performance. There were thousands of complaints
made in the UK to the manufacturers and also to photo magazines.
The magazine "Amateur Photographer" fearlessly took these complaints
to the manufacturers. AP was fearless because, in those days, most of
its advertising revenue came from sellers of used equipment, so it was
nowhere near as dependent on advertising by manufacturers as it is
now.
It had already been discovered by many repair technicians that these
35-80mm lenses shared common DNA, with identical optical and basic
mechanical designs. It didn't take long for "Amateur Photographer" to
establish that all the lenses came from one factory. They were all
made by Tamron.
Did these camera brand manufacturers ever explicitly admit to this? Or
did they explicitly and authoritatively deny it, like Nikon does now?
Why do you think almost every manufacturer of APS-C DSLRs and some
mirrorless cameras offers 18-55mm f/3.5-5.6 lenses with near-identical
optics? Who do you think makes these lenses? Tamron.
But will Nikon or the other camera brands ever admit it,
authoritatively or otherwise?
Please feel free to remain in your Nikon cocoon, protected from the
real world by soothing Nikon press releases and authoritative denials.
;-)