Nx 50-150 is it really F4.2

Some misinformation in this thread.  It is not only the DOF that is different, but also the noise because there is less total light reaching the sensor.  An extensive coverage of the topic is here: josephjamesphotography.com/equivalence

So, yes, in some significant ways it does perform in a way similar to that of an f/4.2 lens on a FF camera, but exactly the same as if you used those FF f/2.8 lenses on an APS-C camera.  But using a Canon 70-200 on their APS-C (1.6x) is like a 112-320mm f/4.2 FF equivalent whereas the Samsung is like a 75-225mm f/4.2 In terms of DOF, but also photon shot noise if the FF and APS-C sensors being used have the same quantum efficiency.

Don't get me wrong, I'm no FF sycophant.  I may have a 5D, but it is a very old camera and my NX1 is far superior.   I am eyeing the 1dxII though, we'll see if it's worth the upgrade to shoot side by with the NX or not.  I prefer the NX system over anything else on the market, but physics is physics and while yes, the lens IS f/2.8, it does not act as an old film FF user would expect an f/2.8 lens to act (and for historical reasons that is some sort of de facto standard, we could just as easily calculate everything in relative terms to any other sensor size, say m4/3) in terms of angle of view, shot noise, and DOF.   Whether you're happy with the physics or not is up to you, I'm quite satisfied with f/2.8 on APS-C for the vast majority of cases.