When I got the S5II I was planning to stick with Panasonic lenses only. I thought that AF would suffer with other lenses, especially video CAF. However, I recently picked up the Sigma 45 f2.8 and 28-70 f2.8 as compact options for one lens outings. I have been pleasantly surprised. Both Sigmas have been rock solid with AF. In practice, I can't really see any difference between these and the Panasonic lenses. I haven't tested every scenario - e.g. I don't shoot burst stills - but basic stills and video have been great. Just wanted to share this in case others had similar hesitations with non-Panasonic lenses.