Lock Exposure XF405

[nilly]

XF405.  I don't want Manual Exposure, I need to be able to let the camera adjust itself, then LOCK all of the exposure functions: aperture, shutter speed and gain, while leaving Auto Focus operating. So if a bright object enters the frame the picture doesn't darken. I've tried reading the manual and fiddling with assignable buttons, custom button etc, and I cannot find a way to do this. 

[IamintheUK]

You can do this in the Manual Exposure mode using Momentary Automatic Aperture. Having a button assigned to this you press and hold  the button so the camera adjusts the exposure. When you let go of the button it will stay at that exposure.

Look at page 70 of the manual..

[nilly]

Thank you for your reply. I assigned a button to Push Auto Iris, but it doesn't seem to work. Is this because I am using external recording on a ProRes monitor/recorder? If so what can I do?