During Avatar i took my glasses off a few times and the screen had that 'double vision' thing going on. I couldn't sit through an entire movie like that.
That's because you are in a 3D screening. In 3D two images (one for each eye) are projected to the screen at the same time, and the glasses have special filters to only show the correct image for each eye. You are not supposed to take off your glasses in a 3D screening.
I didn't even know you COULD go to a 2 D Screening! I thought if a movie was filmed in 3D that was the ONLY way you could see it.
So....you're telling me that if Star Trek is filmed in 3d i will still have my choice of seeing it in 2D in some theaters?
Yes... The 2D version of Avatar (for example), is just one out of the two images. (left or right eye view). That is what the 2D DVD/Blu-ray release will be as well. (until they release a 3D version for Blu-ray)