I just started using Final Cut Pro X and I have footage in 50 fps. I want to play this back in 50% speed in a project that outputs 25 fps. What is the proper way do achieve this? I can start a project with 50fps, import the media an choose "50%" speed reduction, but will this really intepret the media as 25 fps and output 25fps from the project? I suspect that it won't that it infact just will "make up" frames and ouput 50fps, and I don't want that. Any clue on how to fix this would be appreciated.
Your project's frame rate should be the playback frame rate. So if you want to output 25fps, make it a 25 fps project. When you import other frame rates, many NLEs will attempt to automatically adapt the frame rate to your project rate. If FCPX does not do this for you automatically, then you can manually set the timing of the clip to play back at 50%.
But when you make a 50fps project, you are basically making it impossible for FCPX to do the right thing. Setting a clip to play back at 25fps in a 50fps project is meaningless.