i'm watching an excellent behind the scenes video for Star Trek: The Experience. at at one point they get in close on the video assist monitor and it took me a minute to realize that they added crude sketches of the bridge consoles to the safe frame. dudes rock
I shared this with a friend and they had no idea what a safe frame was, so here's a summary.
In actual-film filmmaking, the director of photography (or "cinematographer") has a viewfinder that shows them the exact image seen by the camera lens. It's a purely optical mechanism however, so only the DP can see it. The photo in the first post is of a "video assist", a TV monitor connected to a camera which shares the viewfinder image through a beamsplitter. This makes it possible for more people than just the DP to see the shot, among many other things. Video assists revolutionized filmmaking in countless ways; this is one.
The bright white rectangle is a "safe frame." In short, it's the part of the camera's field of view that can be trusted to always appear in the finished product, and all important action must appear inside that frame. But wait - why is that? Shouldn't every millimeter of the image be usable? Well, in an actual theatrical production perhaps it would be, but this is not that.
Perhaps the most commonly known use of the safe frame is in television productions, where it was used all the way back to the earliest days of the medium. CRT televisions have a property called "overscan" that means that a viewer almost never sees the entire transmitted image. One reason for this is that televisions (in the US; europe is a different story) always had a plastic bezel that hid the outer edge of the CRT, and a significant amount of the image could be hidden behind that.
There was never any standard for the size of that bezel, it was up to the manufacturer; and on top of that, TV tubes were very analog devices, and even two sets of the same model couldn't be trusted to have the same image geometry. Some amount of the image may even have been drawn past the edge of the actual screen, and therefore not visible even if you looked at the bare tube.
In other words, as a DP, you have to assume that a frightening amount of the image - as much as 20% - could be lost for one reason or another. Remember the disaster of the Simpsons' widescreen transfer, where jokes were ruined completely due to the 16:9 aspect ratio cutting off the top and bottom of the frame? The same thing would happen if a DP assumed the entire film frame would end up visible to the audience, so instead, they used a "safe frame", a rectangle 10-20% smaller than the total image.
Safe frames changed when we went from 4:3 to 16:9, but are still around, because for miserable reasons overscan still exists (do not discuss this with me, I know the reasons and I don't care; there were better solutions.) So even on a modern TV production, the ironclad rule is: never put any action that matters outside the safe frame. You can HAVE action out there, and in fact you have to, because once again, overscan varies wildly between TVs for no good reason. Many viewers will even get to see the entire image as-shot, so everything the camera sees has to be "clean", but the only action you can have out near the edges has to be incidental and optional, in case the user has a really shitty TV.
So, going back to the original post - Star Trek: The Experience was not a television production, but it was intended for a very strange venue. The specific scene being shot here is meant to be rear-projected onto a screen representing the viewer on the bridge of the Enterprise-D.
The ratio of that screen is close to 16:9 from the looks of it, so that's problem #1. The footage was shot anamorphically, at a much wider ratio (often called Cinemascope) which is very close to the ratio of an ultrawide PC monitor. On top of that, the edges of the screen are partly hidden behind a rounded facade, meaning there is no way to capture a single rectangular image that will fit it perfectly.
The projector could also become misaligned, or the facade could be altered at some point, so I would bet that the safe frame we see on the video assist - the bright white rectangle - represents a 16:9 area that is expected to be projected well within the boundaries of the viewscreen. So the audience will almost certainly see events outside that frame, but all crucial action must happen inside of it.
But to complicate matters even further, the audience stands at the rear of the bridge set, behind the tactical station. From that perspective, the viewscreen is partly blocked by the ops and helm stations and the performers sitting at them. So for this very unusual kind of shoot, the director and DP have to ensure that nothing important ever happens in the lower left and right corners, even inside the safe frame. This is hard to visualize, so someone stuck a transparency over the video assist and sketched in the complicated shapes they need to work around. Problem solved!
(notably, safe frames are still used in modern all-digital shoots, I just wanted to contextualize the significance of the video assist for this era; I do not know if there was any way to add a complex, custom safe frame like this to the optical viewfinder.)
