on sensor AF is usually more accurate. That's the advantage of mirrorless... no lens calibration required. That's why many EF users are saying old EF lens are working much better on the R. I expect 3rd party EF lens AF to work much better on the R compared to DSLR. I guess this is the value of the R system that Canon has brought in and also banking on. the AF of EF lens on the R should be way better than EF 3rd party adapted option.Personal experience with Sigma 35mm on 5DSR was not that pleasant and focus was consistently inconsistent! However, Sigma 20mm on EOS-R is totally different experience. It seems that later Sigma lenses on Canon mirrorless bodies perform quite well and features such as profile based correction and in camera vignetting removal work fine. I haven't tried Sigma's zoom or longer lenses, though.