Trouble achieving effect without flicker?

[biochemical_animations]

I have an animated scene where I wanted to darken the background objects by excluding them from all lights in order to bring out the foreground focus objects. It creates a nice focal contrast. But I get a flicker (in the RGB and lighting pass) on those background objects. Even after frame blend denoising. Only way to eliminate the flicker is to include the background objects in the lights thereby eliminating my focus contrast effect.

When the background objects are excluded from the lights, they are lit just a bit, but still darkened...I'm guessing the small amount of light they are getting is from bounced light, possible off animated objects. I am using RT CUDA and BF+LC. I've tried increasing the LC higher, but this doesn't reduce the flicker. I'm guessing because the background objects are not getting enough light, vray has trouble somewhere, but I can't figure out a solution.

Does anyone have an idea I could try? I also have noise lock on and retrace at 8.0. TIA

[AlexP]

If all you want to do is darken certain objects for a visual effect, this should be done in post.
Render with full lights as normal, then using masks etc darken the objects you want in post.

[biochemical_animations]

Thanks, Alex.