Each frame of film is a rectangle- each frame is 1/3 wider than it is tall...It's the same shape as your computer monitor or standard TV. (This is referred to as a 1.33:1 'aspect ratio'- since it's the ratio of the width to the height.)
Standard theatrical motin pictures have a wider image- it's nearly 2 times wider than it is tall. (This is the 1.85:1 aspect ratio.)
In order to obtain the 1.85:1 aspect ratio, the entire 1.33 film frame is exposed, but in projection a small area of the top and bottom of the frame is masked off by the projector.
Because the top and bottom are supposed to be masked off in projection, the camera's viewfinders have markings so that the cameraman can compose the image accurately. The very top and bottom of the frame are not supposed to be seen and if you were to see the entire film frame, you'd notice tape marks on the floor, camera tracks and occasional boom mike shadows. These aren't mistakes- the cameraman is well aware of them, but they wouldn't appear when projected...Unless your theater's projectionist has it misframed.
The VHS release of BURNT OFFERINGS was transferred showing the entire frame- the very bottom and top of the image was not meant to be shown to audiences. Right near the end of the film there's a wide shot of the family walking to and getting into the car. If you watch the bottom of the frame, you'll notice a shadow from a boom mike which moves to follow the actors. This isn't supposed to be seen.
You'll notice similar issues with many VHS tapes- the older release of PEE WEE'S BIG ADVENTURE had the worst examples of this. The HOUSE OF DARK SHADOWS and NIGHT OF DARK SHADOWS were filmed in the same format and the VHS releases have similar framing issues, but they're a crane can be seen. Again, these are not not as bad. They don't show the entire frame, it's kind of a compromise. This is why a floor mike, a boom mike and the edge of a crane are visible in NODS. These are not goofs, just improperly framed video transfers.
There are many other aspect ratios, filming approaches and ways of projection, but I'll leave those out of here for the sake of clarity.
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