This link is for a really good lens-to-lens image quality comparison tool. I have not seen anything else quite like it. You can see and compare image quality of 2 lenses side-by-side from pretty much the entire lines of Nikon, Canon, Sigma, Tamron and ZEISS. Plus a few others. You can even see each lens at multiple aperatures and at various zoom lengths.
Shots are of test pattern charts, so you can easily see blur, distortion, chromatic abberations, center sharpness, edge softness, etc..
They include pretty much the entire line of Nikon, Canon, Sigma, Tamron and Zeiss lenses. You can compare various Nikon lenses to each other, or compare Nikon to Canon or Zeiss to Sigma or whatever.
You can even compare the longer lenses with teleconverters mounted, though it is not readily apparent how. Just push the lens past its maximum native focal length and you get to see it with a teleconverter attached.
I see a lot of lens questions posted. I hope this is helpful to someone. Cheers!