I am a bit confused... I thought that a circular filter was only needed for cameras using a semi transparent mirror, and that therefore I could get a cheaper linear filter (allowing me to buy a better one). But I also found one reference saying that PDAF systems required the circular ones. Was the poster simply assimilating PDAF with mirrored systems (DSLRs) vs CDAF for mirror-less? Now that the X100S has a hybrid system, I'd like to know whether PDAF would be affected by a linear filter, or the issue is truly only one of semi-transparent mirrors.