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4K 60fps will be in the pipeline but i don't think we see it on the iPhone 8, there be no 6K it be straight to 8K more than likely and that a good few years off yet due to amount of processing power needed.

4K at 60fps is what Apple & other Smartphone Manufactures need to be aiming for now, which we may see by the time we hit the A12 & Snapdragon 845 & Samsung 10 series Exynos.
 
Its not an impossible task by todays standards however it doesn't make sense until hardware HEVC decoders are commonplace in other devices. Even though you can get phones with up to 256gb of storage its not a very good idea to have a video consuming near a gigabyte of data every minute (4k@60hz) if it can only be played by computers with certain software. Something more elegant is required for a mass consumer product like the iPhone.
 
Even ignoring the fact that the megapixel count doesn't necessarily mean that's how high your video resolution would be since the processor can only handle so much, no, 13 MP would not be enough for "6K" video.

If we go by 2K being 1080x1920 and 4K being double that at 2160x3840, 6K would be at least triple 2K at 3240x5760 which would require a minimum of 18.7 MP. 8K would require at least 33.2MP.
 
What is rumored for the 8's camera improvements?


I would much prefer.

#1. HDR 4K 10-bit HEVC recording it takes a lot of CPU power to record and process data in Billions of colors vs 16.8 Million colors.
#2. 4K video at 60fps
#3. Dual Sensor capture
 
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