Eli5: How can cameras record 4K 60 or 120fps but can’t manage more than around 10fps in photo (or burst) mode (either jpg or raw)?
In: Technology
Recording video is “more efficient” than taking photos. Every photo must contain enough data to reproduce the image. That’s what it means to take a photo, after all. It’s true that you can compress photos, but regardless, you are storing data about what the whole frame looks like. Video, however, is rarely stored simply as a collection of such images. Instead, a first frame is fully captured as expected. For subsequent frames, it is only necessary to record the *changes* from that first frame. Pixels which remain unchanged across frames simply don’t change: you don’t need to “tell them again” to be a color they already are. Imagine shooting an interview: The whole background might never change! The result of this is that, for the same number of frames per second, we expect saving still photos to require a much higher data rate.
It’s also worth noting that cameras these days routinely shoot huge photos, on the order of 20 megapixels. That, all else being equal, further drives up the data rates needed for rapid collection of still photos. Memory can only be written to so fast. In fact, burst mode generally outstrips the ability of memory to be written to and a super fast buffer is needed to pick up the slack. Copying over these buffered shots to memory is (at least part of) what happens when you must wait for a few seconds to shoot again following a burst.
There are a few different limits, which limit applies depends on device and scenario.
Some components are limited by the pixel throughput. So 4K @ 60Hz would be ~500 MP/s. The same component trying to take a burst of 20MP photos could only get ~25Hz.
Another limit is the amount of post-processing done on the images. The optics in cell phones in particular are terrible so images require a lot of processing to look nice (even large cameras with quality optics will do some amount of processing unless you use raw capture.) Videos are obviously required to keep up with the framerate of the video so typically use simpler processing whereas for photos can take longer to look nicer; so cameras do just that and use more processing time on photos than on video or preview. This runs afoul of burst mode, where it will either be limited to the processing rate (if you can burst forever) or limited to a small amount of time you can bust as it only has so much memory to store raw frames for the backlog of processing.
Lastly is the storage rate. Video is highly compressed, so has a better chance of the flash store being able to keep up with it (external SDCards will sometimes advertise the rate of video they can write, internal storage in cameras/cell phones will be matched to the video capabilities of the device.) With something like raw capture though, no compression is used so the file sizes are huge in comparison. Similar to processing, a burst of raw photos often can’t be stored in real time to flash as it is too large, so you can only take a limited time burst until you run out of RAM to store it while waiting for flash to catch up.
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