It's tempting to judge video quality purely by the pixel resolution figure: 1080p is bound to be better than 720p, surely? But while the pixel dimensions are important and determine the physical size at which the video can be viewed, it isn't the whole story. A number of other elements come into play too.
These include the optical quality of the lens, the size of the sensor, and the file formats and types of compression used to record and save the video. The Canon EOS 5D Mk II has reaped the benefits of its much larger sensor, delivering a wider dynamic range than the smaller APS-C and MFT sensors, greater subject detail and more tonal depth.
For ultimate quality, however, a premium L series lens should be used. The adequate but not great kit lenses used on the sub £1,000 cameras here will not have helped their cause, though they're the lenses that 90% of the buyers of these cameras will be using. The Panasonic GH1, in contrast, comes with a ‘kit' lens, but a superb one, designed for video and which accounts for over 50% of the cost of the camera. (The almost identical but video-free G1 comes with a more typical kit lens and costs a third of the GH1.)
Good auto-exposure performance is paramount with video because, unlike stills photography, the camera will need to continually adjust exposure during recording to compensate for changing light conditions, and it must do so smoothly. Colour and contrast can be fine-tuned before recording, but most buyers will expect these to be optimised at the default settings.
As for file formats, it seems that the MOV formats used by Canon and Panasonic deliver a superior quality to file size ratio than M-JPEG, and longer clips too.
Each camera was given a number of subjects to shoot at Kew Gardens in London, ranging from motion to low light and both indoor and outdoor environments. For full video quality head along to our YouTube channel.