If you shoot stills you'll be used to choosing memory cards based on speed and capacity. These are even more important for video, but the requirements are slightly different. Memory card makers will quote a 'maximum' speed which is useful for gauging their performance for stills photography with continuous shooting and picture transfer – for example, '1000x' or '160MB/s'.
But for video you need a minimum SUSTAINED speed, which is not the same thing, so most memory cards now quote both.
With SD memory cards, look for the 'Class rating'. If you're shooting full HD video, you'll need a class 10 memory card. If you're shooting 4K, you'll need a faster card still – for this you need to take a step up to the UHS rating scheme and UHS Class 3 memory cards.
Compact Flash cards use a different rating system called VPG, or Video Performance Guarantee. There are currently two standards: VPG20 guarantees a minimum sustained speed of 20MB/s per second, while VGP65 guaranteed 65MB/s. Typically, you'd chose VPG20 for full HD video and VPG65 for high-quality 4K video.
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