A few DSLR Video cameras output a 1080i signal from the HDMI port for previewing, but the moment you hit record, the HDMI out drops down to 480 SD. Maybe the camera doesn't have enough processing power, or maybe there's some type of conspiracy going on. Either way, most LCD monitors will blank out for a second as it needs to detect the new video feed. A few monitors over the years have added certain features to prevent this, and you just need to find the right mode. In this video i'll show you how it's done on the MustHD M-501 HDMI LCD Monitor.
So if you are shopping for a monitor, and you happen to have a camera (mostly Canon) that drops the HDMI resolution when you hit record, make sure the monitor has a feature like this to prevent those types of drop outs. Hopefully this has been helpful.