D90 : activate Liveview with USB for ManualFocus setting

Dear all,

I would like to make a script for focus stacking with gphoto2 and other 
free software.

My APN is a Nikon D90.

The capture image function works well.

I tried to manage the focus with :

gphoto2 --set-config /main/actions/manualfocusdrive=-50

I have an error saying I must be in Liveview mode to succeed.

My problem is : when I put the USB cable on the D90, the Liveview is 
disconnected ; if I try to activate again the Liveview by pushing the 
Liveview button, it doesn't work.

Do you have a solution for that problem ? How may I use gphoto2 with USB 
cable and get Liveview in the same time with Nikon D90 ?

Thanks,
Regards,
Robert.

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