If a phone can have the shutter speed set to 1/4000 of a second, why can’t it record video at higher than 240fps?
In: Technology
So these answers are partially right and partially wrong. The time it takes to read the data is a few microseconds, which is much much faster than the exposure time anyway, but it has to do with how the camera sensor works. As far as I know all smart phone cameras besides the iPhone IR camera use what’s called a rolling shutter. It’s an electronic shutter of course and is really a CMOS sensor that can be turned on and off to capture light at pixels very fast. It’s possible to have a global shutter that works similarly to a physical shutter by turning all pixels on to capture light at the same time and then reading them all at once, but you start to get problems with noise and therefore image quality and it gets exponentially more expensive to avoid that with more pixels and faster speeds. So rolling sensors instead expose each line of pixels individually, staggering them so that there is time to read each line one at a time. AKA lets say it takes 5 microseconds to read one line of pixels; in your example it exposes each pixel for 1/4000 of a second, or 250 microseconds. It will start exposing the first line, then the second 5 microseconds later, and so on. A modern camera might have upwards of 5000 lines of pixels. Even at just 5 microseconds per line, that means the last line of pixels actually takes a picture 25 milliseconds after the first, and is why taking a picture of spinning fan blades and other fast moving stuff looks weird on a phone camera. For video it makes each image one at a time so it has to wait until all of the pixels are captured and create an image for each frame. In the above example you expose pixels to light for 1/4000 of a second but it actually takes 1/400 of a second to capture the whole image/frame, plus a bit of time to “put together” and store the image.
A phone can take a still image with that shutter speed, but to do it more than 240 times per second also means you have more images to store on the phone. Moving those images from the sensor, to the the phone memory, can be a bottleneck to performance. Even high end dedicated DLSR or mirrorless sports cameras like the 20 frame per second Sony A9 have limitations because they have a bottleneck moving the photos onto the memory cards. Very high frame rate video cameras exist but of course the faster they are, the more money they cost. There are other complications also. For example, capturing an image at 1/4000th of a second shutter speed usually requires a lot of light. If you watch the slo motion channels on youtube, you may notice the super slo motion clips are darker, because they are using such fast shutter speeds. I hope this helps.
Theoretically it can it is just that there is little practical use for it since the human eye can’t register images that fast so unless you are going to record a super slow motion film there is no point.
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