Frame rate and shutter speed are entangled fundamentally.
Frame rates put a cap on shutter speeds. At 24 fps, the fastest you can go is 1/24s. At 120 fps, the fastest you can go is 1/120s.
Start first with the aesthetic aspect of motion blur.
Most film is shot relative to a rotary shutter [1], with a 180 shutter being the norm, which is effectively half the frame rate. This is essential to the look of cinema today. Anything longer feels streaky and prone to feeling a bit drunk (see open shutter films by Dante Spinotti etc) and anything shorter feels like it becomes too sharp and soap opera like (see the first hobbit).
The second hobbit film went to great lengths to make the motion blur feel more like 24fps motion blur. At 48fps you have to shoot open shutter (1/48s) to match 24fps style motion blur. Any higher than 48fps and you cannot match the 24fps look.
Now this is a purely subjective choice, as is 24fps today ,given we don’t rely on optical audio track,, but it’s ingrained in people for the past 100 years. It’s the cinematic look.
Then there’s the practical aspects.
Firstly is lighting. Increasing frame rate puts a cap on shutter open time. That’s significantly more light that’s needed for every shot. This greatly increases the complexity of films where we’re already using massively high power lights. That means a ton more heat and expenditure.
Then there’s the heat of the cameras. Filming is brutal on cameras. As you increase frame rate, you increase the load significantly, leading to potentially much shorter shooting times.
Beyond that, there’s the expenditure of more frames in general. Doubling frame rates also doubles cost for storage, and the cost of post effects.
And for what benefit really? As much as I like some aspects of 48fps, audiences have pretty universally panned any shifts away from 24fps as unnatural.
Thanks for the reply. I really do appreciate it, because coming from the still photography world I've never understood this.
Not to be pendantic, but you can't you achieve motion blur by blending frames? That is, shoot at 1/240 and either average or add 10 frames together in an overlapping sequence to get 240 FPS with motion blur that looks like 24 FPS?
I'm only half convinced about lighting. Certainly, if a great deal of light is already required, then running a faster shutter speed means you need more light. That's basic physics. But I've been doing still photography indoors, with horrible lighting, and often I don't need to shoot any slower than 1/90 or so (and often can't, depending on the lens I'm using, since I shoot handheld). Maybe I'm using a larger aperture? Not sure what gives because with modern digital camera bodies, sensors have gotten really, really good.
Blending frames only works in very few scenarios. It’s not something that scales well to the multitude of things that need to be shot. For example, If something enters the frame, how do you blend backwards without a frame to do so?
Frame interpolation and generation ignores the contents of the screen and softens all elements, resulting in the overly mushy results you see on TVs with motion smoothing turned on. Impacts are softer, lip syncs may be off.
To your other question, even with your stills camera, you compensate for your higher shutter speed by changing exposure elsewhere.
Either you need a wider aperture, but now you’ve set limits on your depth of field, and potential sharpness. This is part of storytelling, for both stills and motion.
Or you’ve added more noise by bumping sensitivity. Which people don’t notice as much on a still, but do with subsequent frames when noise isn’t spatially and temporally stable. You’re also not driving your sensor for extended periods of time with stills where that heat buildup of extra sensitivity can matter.
Or you need more light.
The camera is one of the most emotional parts of a film. Yes you can do all these other things, but by doing that you place hard limits on the story the camera itself tells.
Comments
Frame rate and shutter speed are entangled fundamentally.
Frame rates put a cap on shutter speeds. At 24 fps, the fastest you can go is 1/24s. At 120 fps, the fastest you can go is 1/120s.
Start first with the aesthetic aspect of motion blur.
Most film is shot relative to a rotary shutter [1], with a 180 shutter being the norm, which is effectively half the frame rate. This is essential to the look of cinema today. Anything longer feels streaky and prone to feeling a bit drunk (see open shutter films by Dante Spinotti etc) and anything shorter feels like it becomes too sharp and soap opera like (see the first hobbit).
The second hobbit film went to great lengths to make the motion blur feel more like 24fps motion blur. At 48fps you have to shoot open shutter (1/48s) to match 24fps style motion blur. Any higher than 48fps and you cannot match the 24fps look.
Now this is a purely subjective choice, as is 24fps today ,given we don’t rely on optical audio track,, but it’s ingrained in people for the past 100 years. It’s the cinematic look.
Then there’s the practical aspects.
Firstly is lighting. Increasing frame rate puts a cap on shutter open time. That’s significantly more light that’s needed for every shot. This greatly increases the complexity of films where we’re already using massively high power lights. That means a ton more heat and expenditure.
Then there’s the heat of the cameras. Filming is brutal on cameras. As you increase frame rate, you increase the load significantly, leading to potentially much shorter shooting times.
Beyond that, there’s the expenditure of more frames in general. Doubling frame rates also doubles cost for storage, and the cost of post effects.
And for what benefit really? As much as I like some aspects of 48fps, audiences have pretty universally panned any shifts away from 24fps as unnatural.
[1] https://en.wikipedia.org/wiki/Rotary_disc_shutter
Thanks for the reply. I really do appreciate it, because coming from the still photography world I've never understood this.
Not to be pendantic, but you can't you achieve motion blur by blending frames? That is, shoot at 1/240 and either average or add 10 frames together in an overlapping sequence to get 240 FPS with motion blur that looks like 24 FPS?
I'm only half convinced about lighting. Certainly, if a great deal of light is already required, then running a faster shutter speed means you need more light. That's basic physics. But I've been doing still photography indoors, with horrible lighting, and often I don't need to shoot any slower than 1/90 or so (and often can't, depending on the lens I'm using, since I shoot handheld). Maybe I'm using a larger aperture? Not sure what gives because with modern digital camera bodies, sensors have gotten really, really good.
Blending frames only works in very few scenarios. It’s not something that scales well to the multitude of things that need to be shot. For example, If something enters the frame, how do you blend backwards without a frame to do so?
Frame interpolation and generation ignores the contents of the screen and softens all elements, resulting in the overly mushy results you see on TVs with motion smoothing turned on. Impacts are softer, lip syncs may be off.
To your other question, even with your stills camera, you compensate for your higher shutter speed by changing exposure elsewhere.
Either you need a wider aperture, but now you’ve set limits on your depth of field, and potential sharpness. This is part of storytelling, for both stills and motion.
Or you’ve added more noise by bumping sensitivity. Which people don’t notice as much on a still, but do with subsequent frames when noise isn’t spatially and temporally stable. You’re also not driving your sensor for extended periods of time with stills where that heat buildup of extra sensitivity can matter.
Or you need more light.
The camera is one of the most emotional parts of a film. Yes you can do all these other things, but by doing that you place hard limits on the story the camera itself tells.
I feel like I've taken college courses that taught me less than your series of comments here. Bravo, fellow HN'er.
Haha, I appreciate it. Perhaps the perks of having worked both on set and post production , and both on the creative and technical side.
Did you mean, at 24 fps the _slowest_ you can go is 1/24 s, and at 120 fps, the _slowest_ you can go is 1/120 s?
HFR sequences in avatar2 were weird at first, but after 10-15 minutes became wonderful and caused the 24 FPS ones to feel jarring.