jackpinoh Loc: Kettering, OH 45419

I think it is a mistake to generalize as you have done. It depends on the specific lenses being compared, the currency of their technology and things like AF speed, widest aperture and weather sealing.

Soul Dr. Loc: Beautiful Shenandoah Valley

No matter how much knowledge or technique one has when when using a camera, it will not make a lens with problems work any better than it can.
A lens may have back or front focusing issues, CA problems, pin cushion, barrel distortion or any other short comings.
So, one does have to worry about equipment if one wants to get the best images possible.

BudsOwl Loc: Upstate NY and New England

I have been a long-time subscriber to PhotoPlus the Canon Magazine published in the U. K. It often reviews lenses and selects a “best on test” and a “Best Buy”. Sometimes it picks the Canon over all the others, but at other times it picks either a Sigma or a Tamron. It is one of the two photography mags I subscribe to, the other is Outdoor Photography.
Bud

lamiaceae Loc: San Luis Obispo County, CA

Theoretically what you are saying can make perfect sense, but is that what is really happening (if anything), I could not say. And how many really could know unless you actually had all the products under discussion and scientifically, correctly, and accurately measured or evaluated them yourself. But yes, I certainly could by into some companies are "closer" to one another and may be more cooperative with sharing info. Example Intel & Microsoft! There certainly are differences in third party lenses and different brands of cameras. I see few Sigma lenses for Pentax for example, but quite a few Tamron ones. Personally I'm rather brand faithful and of the 33 or so "Pentax" lenses I own only two are not smc-Pentax Branded, one Tamron (18-200mm) and one Vivitar (19mm). You do know that thru the years many companies have actually built/assembled lenses for Canon, Nikon, Minolta, Pentax, etc. I've heard the first Canon lenses were actually made by Nikon's parent company. Some of these mystery manufacturers are the well known third party companies of the day like Tokina, Vivitar, Schneider, etc.. Other times they are other well known lens and camera makers. Carl Zeiss and Leitz (Leica) come to mind.

Architect1776 Loc: In my mind

Canon and Nikon actually worked together with Canon making bodies and Nikon lenses.
It is a bit of interesting history.
http://www.mir.com.my/rb/photography/companies/nikon/htmls/nikon_canon.htm

dsmeltz Loc: Philadelphia

Yeah, I read motor trend and consumer reports for their entertainment value. It is like Fox News. The shows are funny, but largely devoid of any real world usefulness.

SuperFly48 Loc: NE ILLINOIS

Have had four Tamron lenses for my D7100 and D850. My personal experiences, they do well but are not quite as sharp as the Nikon lenses I use and have used. My first trip to OR back in 2014, decided to take a Tamron 18-270 instead of my 18-200 Nikon DX VR. The shots I got were fine and acceptable, but I had to sharpen most all that I wanted printed. That trip soured me on that Tamron lens. At present I have a Tamron 150-600 that I have used on both the D7100 and D850; on the D7100 I have gotten some pretty good results, on the D850 I have been disappointed with over 75% of the shots taken. Go Figure!!!