The Canon 16-35mm f/4 IS is terrible at 35mm and Infinity Focus

I have hiked my Canon 16-35mm f/4 IS hundreds of miles and it has given me many of my favorite images.  I have mostly used it on the wide end since I usually carry a normal range zoom as well.  On a recent trip, I took only this lens and discovered just how poorly it performs at 35mm and focused at infinity.  Here is a center crop at f/8 and 35mm shot on a 5DS:

No problems here.  About 75% to the edge of the frame, however, is a disaster:

I am fairly careful about picking lenses that will perform well enough on a 50 megapixel camera, so I was pretty surprised.  Now what went wrong here?  Lets look at Canon's MTF charts:

These charts would suggest really good performance across the entire frame, clearly very different that what I was seeing.  It is possible that I simply have a badly decentered copy, but in that case one side of the frame should be better than another, which is also not the case.  Another possibility is that these MTF charts are for a close focal distance and that when focused at infinity, the lens performs entirely differently.  The-Digital-Picture hosts MTF charts for a variety of lenses tested at infinity.  The 16-35mm @35mm and f/8 performs as:

Credit to the-digital-picture and Olaf Optical Testing supported by LensRentals

There we go, mystery solved.  The 16-35mm @35mm and f/8 is simply bad in the tangential MTF at infinity focus and it is readily apparent in photographs.  This is not the first time I have been screwed by Canon's lenses being optimized for closer focal distances.  From a marketing perspective, this makes sense.  The vast majority of optical testing is done by taking images of test charts.  The moral of the story is that one measurement tells you very little about a lens