EDIT: I think I found the problem, an Environment Light that was not enabled in viewport. But do I need to set anything to render out to retain the same ACES colour space for use in Nuke, or will it do so by default?



When I render with Mantra in ACES colour space the render looks very bright, unlike the viewport representation -- see image:

https://i.postimg.cc/k5b8gWpg/ACESManta.png [i.postimg.cc]

I have set the Render Working Space to ACEScg and View Transform to ACES 1.0 - SDR Video

Is there a setting in the Mantra node I need to look at?

I have not had the same problem rendering the same thing in Karma.