I'm playing with my brand new Z1 and have found that when playing with the iris control knob there is a bit more resistance after turning it a bit and then it gets easier again and then more resistance. Is this normal?

Not that I'm aware of.
I use both the Z1 and the FX1 and there's never been anything other than a fixed "feel" to the action of the knurled knob..

Robin

Never noticed that on my Z1...