I have both a Canon F1n and a Nikon F3HP with a variety of lenses. I was a dedicated canon user since the early 1970's, now I have very many SLR's with accompåanying lenses.
I have noted absolutely nothing of that sort.
Nikon, Canon, Pentax, Olympus, Praktica, Praktina, Konica, Olympus Pen - ALL still give marvellous definition and contrast, given that I do my part, and given that the lenses are kept CLEAN.
That what you write is balderwash in my book, and a thin disguise to continue the everlasting "debate" over which is better, Canon or Nikon.
Ever considered a Leica? I have one of those too FYI.........
Going back a lot of years, my loft used to be filled with years and years of photomagazines, english, american, german, swedish, danish, norwegian, even australian from a lengthy visityh in my young days.
All where filled nearly to the brim with tests. Some tests where faulty, some tests was obviously bought. But the best was Modern Photography (USA) and a couple of german magazines, I think Color Photo was one of them.......
General consensus at the time when Canon launched their Flex-1 project and that bombshell lens launch (biggest simultaneous lens launch ever?) was that Canon had the upper hand, and several years of adavantage, which was later demonstraded in lens tests during most of the 1970's, Pentax Takumars was super, Olympus OM was super, Minolta was super, Nikon had a lot of catching-up to do. And more often Canon came out on top by a slight margin.
Nikon might finally have caught up during the late 1980's, it was a time of advances in many fields, super coating and wide-spread use of desk-top computers being the driving force, I personally think that those did most difference when it came to zoom lenses, and think that the wide-angle to short tele zooms we now enjoy would have been impossible witout.
But Nikon being significantly better than Canon lenses, if you compare apples to apples? (lenses in the same price bracket).
NO WAY.
If you don't get sharp pictures from a SLR, most any SLR, made after 1970, either the lens is faulty and in need of repair, or the camera body is.
If your'e still not satisfied, go out and make some pictures, instead if photograping printed board with geometric figures on them.