In order to record video on internal memory cards, cameras have to compress it first, and the degree and type of compression can cause movement artefacts, loss of detail and other kinds of image degradation. In general, the camera's 'bitrate' – the rate at which it can save data – is a good guide to the quality and compression of the video.
But there is a way round this. Many cameras can save 'clean', uncompressed video straight to an external HDMI recorder via their HDMI port. This is increasingly being seen as a strong selling-point for higher-end cameras. Some models can record compressed footage to an internal memory card and uncompressed video to a recorder simultaneously.
