Regarding levels filter: You can create a sample frame (or a sample video) which will show some form of histogramm. This helps a lot to figure out the best settings.
Usually move the upper white thingy to the left using the histogramm and the lower black bumper to the right (as the crushed blacks seem to be on the left adn the whites on the right).
Can't explain why and how this really works and what it means - but it works if you play around with it.
Examples
Original:
Way to dark and to many blacks. Half of the screen is just black.
Adjusting:
Way to extreme settings
This looks way better now - you can now make out some details about the guys hair. Still some crushed blacks in there.
But - this was only analyzing one frame. Jumping to another scene using the same "good" settings:
To bright. And the histogramm is different. All details on the hat are lost now.
So you should either use reasonable settings - or you'd have split the movie into scenes. Some scenes might be to dark and others might be to bright.
Depends on the source. Especially a problem in comps. And with plantinum blondes. I'm to lazy to put that much effort into it. And in this case it's probably better to lose details on sky, white hats and shirts then on the girls
You can create a histogramm about the full movie, but this won't help much (and takes some time) as it will just combine all scenes.
But levels filter can help a lot to improve the source. Think in this case the crushed blacks are are result of my cheapo china stick.