To be clear, there are significant lens flange-to-sensor distance differences between dSLR lenses and mirrorless lenses. The shorter flange distance on mirrorless designs, due to elimination of the mirror box, enables better optical designs, especially for wide angle lenses and short zooms. Lenses specifically designed for mirrorless bodies are likely to perform better than their older dSLR versions, both because of the shorter flange-to-sensor distance, and due to improvements in computer-aided design, lens coatings, and lens manufacturing processes.

All THAT said, dSLR lenses that are correctly adapted to mirrorless bodies should perform as well OR BETTER than they did on dSLRs. Most people notice an improvement in autofocus performance over what they had on their older dSLR bodies. Optical performance should be the same, although better sensors in the mirrorless bodies may eke out a bit more image quality from an older lens.

When mounting some older lenses on very high megapixel count cameras, you reach a point where the lens cannot resolve what the sensor can record. That is where newer, better, designed-for-mirrorless lenses excel. At 24MP, you probably won't have issues with older dSLR lenses. At 60+ MP, you probably want the latest, greatest glass you can find. OR, you need to examine your reasons for buying a 60 MP camera, to be sure they are sound! (Bias-ply tires don't work well on a high end sports car, do they?)

To be clear, I'm not a pixel peeper like so many others. I don't care about the mechanics of how it works as long as it works. I don't own a 60 megapixel camera. My highest megapixel camera is my Canon 5DSr, and it doesn't get used as much as it used to. I don't care about flange distance and stuff like that. I only care that the camera lens combination works. That's really all that matters, and as I said, I have no problem with my Sigma or Canon EF mount lenses working on my R5 and R7 with an adapter.

To be clear, I'm not a pixel peeper like so many others. I don't care about the mechanics of how it works as long as it works. I don't own a 60 megapixel camera. My highest megapixel camera is my Canon 5DSr, and it doesn't get used as much as it used to. I don't care about flange distance and stuff like that. I only care that the camera lens combination works. That's really all that matters, and as I said, I have no problem with my Sigma or Canon EF mount lenses working on my R5 and R7 with an adapter.