In an extensive interview at Variety, James Cameron has a lot to say about 3D production, but he also mentions the paper tiger that is 4K resolution for movies:
4K is a concept born in fear. When the studios were looking at converting to digital cinemas, they were afraid of change, and searched for reasons not to do it. One reason they hit upon was that if people were buying HD monitors for the home, with 1080×1920 resolution, and that was virtually the same as the 2K standard being proposed, then why would people go to the cinema?
He suggests that instead of having 4K (a 4096×3112 frame) 24 times a second, it’s better to go for 2K (2048×1536) 48 times a second. This would reduce the motion artifacts seen at 24 fps. ‘Motion artifacts’ most often happen when the camera pans too quickly – a juddering effect when 24 frames every second isn’t enough to show all the detail we would normally see if we turned our heads at the same rate.
[For those of you who are used to 2K being 1920×1080 and 4K being 4096×2160, I’m referring to the resolution of the full 35mm frame, which is cropped down for different aspect ratios when projected. Wikipedia has more on this.]
Artifacts also occur when objects such as cartwheels and hubcaps have detail that rotates at a rate that is close to 24 frames a second – as a picture is taken every 24th of a second and the pattern looks very similar avery 24th of a second, it looks as if the pattern hasn’t moved far and that the wheel is moving very slowly even though the cart or car is moving quickly.
If regular patterns are close the frame rate, you get strobing. The upper wheel is moving three times slower than the lower wheel.
The spokes in the lower wheel are moving so fast that they rotate almost as far the distance between two spokes, which makes it seem as if the spokes are moving backwards. You can see from the broken spoke that the wheel is still moving forwards.
Juddering pans and strobing wheels still occur at 4K. 4K gives us a more accurate representation of these effects. 2K twice as often will reduce these effects a great deal.2K at 48fps is better than 4K at 24fps. Temporal resolution is more important than visual resolution. This is why interlacing has survived into the digital era – those who want to show sport insisted in rates of 50 or 60 frames per second for their broadcasts. Due to bandwidth limitations, the would rather have half the vertical resolution (1920 by 540) twice as often.
Another advantage is the data rate for storing and communicating the footage would be less: 24 times 4096×3112 is 306 million pixels per second whereas 48 times 2048×1536 is 151 million pixels per second.
Cinema owners may have to let go of beating in-home systems using visual technology, they’ll have to concentrate on the architectural and social elements of a big night out at the movies.
If 2K at 48fps is adopted the post process will need to produce content that can be generated at both frame rates. 24fps has been a standard for so long that it will take years for projectors around the world to be replaced with digital projectors. As it costs $1,500 to produce a film print at 24fps, the sum will almost double for 48fps. With reel breaks happening twice as often, film projection at 48 fps isn’t worth the benefits of providing extra temporal resolution.
This isn’t that much of a big deal for editors. If we treat the extra frames per second in the same way we (used to) deal with interlaced footage, there shouldn’t be too much of a problem. Timecode can stay the same. We’ll stick to making edits only on 24ths of a second. If a 48fps movie is being mastered, it’ll get a bonus frame at the end of each shot. We’ll probably edit away at 24fps for now. Once the edits have been agreed on, we’ll be able to watch at 48fps to see if any moments added at the end of a shot are undesirable. We can then move the edit back a 24th of a second if need be.
It won’t take too much effort for Avid, Apple and Sony to add features to enable 24/48 fps workflows in their software. The sooner they do, the better the fidelity of the movies we make.