Hasta La Motion!

I have a problem I cannot solve. I have an Apple ProRes 422 HQ 8 bit mov (let’s call this "Original").
I simply Read it with default 1.8 gamma.
I connect a Write node after that to write the exactly same Apple ProRes 422 HQ 8 bit mov with 1.8 gamma. (call this "Converted")

Now I read the Converted mov back. I can’t see any difference between Converted and Original in Nuke’s Viewer (swapping back and forth). If I add a Merge node with difference selected, there is a visible difference, unless I turn video colourspace on. If I do, it’s OK.

Now if I open both in QT Player and swap back and forth again, the Converted is definitely lighter than the other one.

Nuke 6.1v2 and OSX 10.5.8 here.

Any ideas what is going on here? How can I fix this issue?

Thanks,

pH.

ps: easy on me please, I’m new on Nuke..:)