24 fps was the capture rate of old film cameras. The reason for that is because of how sound was stored on the edge of celluloid tape. Because of how the sound waves where imprinted on the tape, the max fidelity was at sample rate of at most 10,000 Hz per 18inches of tape. So, 18 inches of tape has to move through the projector and sound head every second.
Consider a 35mm film reel which was popular at the same time. On a 35mm reel, the height of a single frame is about 19mm. So we have 18 inches (457mm) going across the projector screen, which shows a new frame of film every 19mm. That tells us the frame rate: 457/19 = 24.
And that is why films run at 24 fps. Because it was the natural result of technical constraints when sound started being added to film. This is why older silent movies could use lower frame rates. Super 8 cameras recorded at 18fps because they could get away with it, but once Sound Super 8 hit the scene, it got brought back up to 24fps for much the same reasoning as the 1930s had with 35mm film.
But film projectors, even in the late silent era, didn’t just flash the image once per frame. They had two or three bladed shutters so the image on screen would actually flash 48 to 72 times per second even though the film frame was only advancing 24 times per second. The whole point of the extra shutter flashes was to push the flickering of the light on screen above the flicker fusion threshold of your eye where you are able to differentiate between individual flashes of light. For most people, it’s about 50-90 Hz in the center of their field of vision.
Strangely enough, the flicker fusion threshold in the center of your vision is lower than at the edges, which is why you can usually notice the blinking of LED headlights more easily in your peripheral vision than when you’re looking straight at them.
Hmm… I’m detecting that we may be operating with different definitions - made no more clear by the fact that you are talking about projectors while I’m answering a question about cameras. To me, a frame is a single still image on a reel of film.
I think I see your point that a projector would insert more black transitions from on frame to the next so that the black transition from one frame to the next isn’t as noticeable. However, I don’t see the connection to original discussion about frames per second or where the film industry standard of 24fps came from.
The original discussion was whether the human eye sees more than 24 frames per second, and the answer is that it absolutely does. Your context about where the 24fps standard came from is correct, but in the context of the broader discussion, it’s relevant to point out that throughout the 20th century, nobody ever watched those films on displays that refreshed 24 times per second.
Until the the telecine allowed for television broadcast of film in the 1950s, projectors were the ONLY way to view motion picture film, hence their relevance to the discussion. In the 1800s, there were single-bladed projector shutters that flashed the image once per frame and the flickering was highly noticeable and distracting, which is how movies got the nickname “flicks.” The multi-bladed projector shutters give the screen an effective refresh rate of 48-72 Hz, despite the image only changing at 24 Hz. Even when films started being broadcast on TV via telecine, TVs in the NTSC region were operating at 60 visual field scans per second, meaning that 24fps film frames had to be shown on an alternating 3-2 repeating pattern to fit the 60Hz scans.
24 fps was the capture rate of old film cameras. The reason for that is because of how sound was stored on the edge of celluloid tape. Because of how the sound waves where imprinted on the tape, the max fidelity was at sample rate of at most 10,000 Hz per 18inches of tape. So, 18 inches of tape has to move through the projector and sound head every second.
Consider a 35mm film reel which was popular at the same time. On a 35mm reel, the height of a single frame is about 19mm. So we have 18 inches (457mm) going across the projector screen, which shows a new frame of film every 19mm. That tells us the frame rate: 457/19 = 24.
And that is why films run at 24 fps. Because it was the natural result of technical constraints when sound started being added to film. This is why older silent movies could use lower frame rates. Super 8 cameras recorded at 18fps because they could get away with it, but once Sound Super 8 hit the scene, it got brought back up to 24fps for much the same reasoning as the 1930s had with 35mm film.
But film projectors, even in the late silent era, didn’t just flash the image once per frame. They had two or three bladed shutters so the image on screen would actually flash 48 to 72 times per second even though the film frame was only advancing 24 times per second. The whole point of the extra shutter flashes was to push the flickering of the light on screen above the flicker fusion threshold of your eye where you are able to differentiate between individual flashes of light. For most people, it’s about 50-90 Hz in the center of their field of vision.
Strangely enough, the flicker fusion threshold in the center of your vision is lower than at the edges, which is why you can usually notice the blinking of LED headlights more easily in your peripheral vision than when you’re looking straight at them.
Hmm… I’m detecting that we may be operating with different definitions - made no more clear by the fact that you are talking about projectors while I’m answering a question about cameras. To me, a frame is a single still image on a reel of film.
I think I see your point that a projector would insert more black transitions from on frame to the next so that the black transition from one frame to the next isn’t as noticeable. However, I don’t see the connection to original discussion about frames per second or where the film industry standard of 24fps came from.
The original discussion was whether the human eye sees more than 24 frames per second, and the answer is that it absolutely does. Your context about where the 24fps standard came from is correct, but in the context of the broader discussion, it’s relevant to point out that throughout the 20th century, nobody ever watched those films on displays that refreshed 24 times per second.
Until the the telecine allowed for television broadcast of film in the 1950s, projectors were the ONLY way to view motion picture film, hence their relevance to the discussion. In the 1800s, there were single-bladed projector shutters that flashed the image once per frame and the flickering was highly noticeable and distracting, which is how movies got the nickname “flicks.” The multi-bladed projector shutters give the screen an effective refresh rate of 48-72 Hz, despite the image only changing at 24 Hz. Even when films started being broadcast on TV via telecine, TVs in the NTSC region were operating at 60 visual field scans per second, meaning that 24fps film frames had to be shown on an alternating 3-2 repeating pattern to fit the 60Hz scans.