Sony Z1P is set to manual, but the iris keep changing

On the Z1P and many other digital video cameras, you must be sure that  ALL OF THE MANUAL ADJUSTMENT icons are visible in the viewfinder and/or LCD if you want to have full manual control over your images.

Iris, Gain, Shutter Speed and White balance settings and icons must all be in seen along the bottom of the screen. Do this by pushing the corresponding Iris, Gain, Shutter Speed and White balance buttons on the camera.

Here is why

The camera software assumes that any parameter that you don't see on the screen must be automatically adjusted by the camera. For example, if you have all of these settings active except the GAIN, the camera will automatically make a gain level adjustment for you when the picture is too dark. Even though you have control of the IRIS, it appears to be adjusting the light level automatically when panning to dark area of the scene. 

It's not the IRIS that is automatically adjusting here, it's the GAIN.

Be careful not to accidentally bump the SEL/PUSH/EXEC wheel at the rear of the camera as it always remains "active" on the shutter speed adjustment after all the other adjustments are set.

Finally, remember, "locking" the manual adjustments by sliding the switch to the HOLD position locks out ALL the adjustments, including the iris, which is the one thing that you usually need to have control over.
Pieter de Vries ACS