When you read the camera specs you'll also see the frame rate figure (or sometimes the resolution figure) is followed by a 'p' or an 'i'.
'P' stands for 'progressive' video, which means that the camera records a full frame at a time. This is 'proper' resolution and delivers the best quality.
'I' stands for 'interlaced', and it's a bit of a kludge where one frame actually consists of only half the image lines (in old fashioned frame scanning jargon) and the next consists of the other half. These count as two 'fields' but it's only when they are 'interlaced' that you get a complete new frame.
Interlacing video saves bandwidth when broadcasting video and it reduces the processing demand on the camera. The Nikon D5200, for example, can shoot full HD video at 30p and 60i – at 30fps the video is 'progressive, but to get to 60fps the camera has to switch to interlaced video.
You don't see interlaced video so much now – camera processing power is reaching the point where it's not necessary.
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