Alex Cooke at Fstoppers has decided to try and tackle some of the myths and truths about sensor resolution and lenses. If you buy a Canon EOS 5DS, or EOS 5DS R, do you need to be aware of what lenses you use with them to get maximum performance?
It's a tough question to answer and is difficult to quantify with any current testing methodology and in the end may be up to you.
From Alex Cooke:
There’s also an added complication independent of the physics: we don’t normally see lower level glass paired with higher level bodies. This in itself has normally kept a certain level of separation, but now, as megapixel counts approach unforeseen levels, that gap is being closed. And so, we have to ask: if you buy a ultra-high-resolution camera, will even your good glass be rendered obsolete? Well, I can’t answer that for two reasons: first, it depends on your definition of “obsolete.” If you have a 50 megapixel sensor, are you willing to take a hit of 5 megapixels before you call a lens archaic? How about 10 megapixels? That’s up to you. Second, at this point in time, we don’t have data obtained from a publicly available, verified method to make that call. My suspicions say that indeed, the threshold of degradation may be starting to cross into lower level professional lens territory, but they're just that: suspicions. Read the full article
Lenses are going to be updated to the latest technology for both high megapixel DSLRs as well as 8K and beyond for cinema lenses. If you're shooting an 50+mp camera, it can't hurt to own the latest version of a lens, but don't be worried about using older and less expensive lenses, they're still going to give you great results.