The viewfinder gets a fair bit darker compared to the stock focus screen if you're using slow lenses (f2.8 and faster is the same, f4 is noticeable, f5.6 is much darker). Other than that, no issues at all.I can offer you a much better (and cheaper) solution. First, if your camera takes interchangeable focusing screens (all canon FF cameras except 5D3), install screen Eg-S, which will make subjects really snap into focus vs the stock screen. Zeiss lenses have focus confirmation, meaning even though you rack focus manually, you still get a focus confirmation light / beep. Focus confirmation via the camera's AF system is just as accurate as focusing via live view sans tripod. Enjoy your Zeiss!
Interesting, does using the Eg-S with EF lenses lose you any functionality?
cheers, good to know that
There's also the Katzeye focus screens, which have the same focusing ability as the Eg-S screens, but also have a split prism and a micro prism collar, much like old film SLRs had. I have one, and the micro prism is the most helpful part, in my opinion. The split might distract some people