I have filmed a cave scene with some actors using green screen and have the actors holding unlit torches. The idea is I need to get an effect where the torches look like they are on and the light is moving around the cave as the actors are waving them around.
Does anyone know how this could be achieved using After Effects?
Hard to say. It's on greenscreen? So are you building a cave in 3D, or in AE from 2D layers arranged in 3D space? Or will the cave be 'invisible', with essentially a black background?
First thing you will need to do is track the torches (this might have been easier if the torches were on). It now occurs to me that by torch you might not mean what we British think of as a torch - i.e. a flashlight. If you are using Indiana Jones style torches you will have a lot more trouble - you will need to create a flame and have the light flickering. There is very little chance this will look real, as you will probably want to have the torchlight interacting with the surface of the actors' bodies, creating light and shadow on a 3D surface.
If you mean torches as in electric lights that Americans call flashlights following Simon's suggestion you could use Trapcode's Lux plug-in in place of T-Beam if you've got it...
However - also agreeing with Simon here - without the proper play of light across the actors' faces, it's going to look very unrealistic.
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Thanks for the feedback. I had a feeling when I shot the footage I should have kept the flashlights on. The idea was for a short advert with some students that I was helping out but have since given up on that. I have had a look at your youtube stuff and as a newbie to AE CS3 I was wondering if you could advice me on the best place to start with it.