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Thanks Philippe and Andrew!
Andrew, I'm not sure, what you want to say with your comment.
If I shoot landscapes from a tripod that are far away with no close foreground in the image I use either f/5.6 or f/8. Everything would make no sense in my opinion.
The same is true if I have something in the foreground and want everything within DOF (again shot from a tripod).
In Croatia I've also shoot many image at f/2. f/2.8 or f/4 with the Summicron. But only in situations, where the camera was handheld or I wanted a limited DOF to emphasize the foreground.
I don't think that it makes any sense to always shoot wide open, just because a lens is designed (and priced) with high performance wide open.
Regarding the RX100: The center sharpness at f/1.8 is very good. In my opinion comparable to the Summicron. But I've not done any systematic testing yet. The images in this thread are just some vacation shots with both cameras.
At the moment I'm trying to return the RX100 to Sony, because of the decentered lens, but I missed the 30 day return period by 4 days, therefore the CS could not decide this directly and will contact me again.