I investigated further and it seems that libgphoto2 <= 2.5.23 suffer from a bug for Nikon camera preventing to capture images longer than 5s, this is fixed on 2.524 ( www.gphoto.org/news/ ). I then compiled from source the newest version of libgphoto2 and uninstalled the old one (without breaking the deps) and now the capure of a 30s shot seems to work fine.

I will update if further problems appear.

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