The Canon Vixia HG21 sports a 1/3in CMOS sensor and the sensor size has a positive effect upon the image quality. In low light and especially in well-lit situations, the HG21's video is of high quality and the image is remarkably clear for a consumer camcorder. I've seen camcorders that have smaller sensors, but the results suffer.
The Canon Vixia HG21 supports 1,920-by-1,080 resolution at 24p Cinema Mode (24 frames per second), 30p Progressive Mode (30 fps), and, of course, 1080i. The p in 24p and 30p means that the camera uses progressive scan video, where video is drawn from top to bottom in one pass.
Many camcorders, instead of progressive scan, use interlaced frames (the i in 1080i), where each video frame is displayed in alternate fields and horizontal lines are displayed from top to bottom. Interlaced video can look jaggy on a computer screen; look for progressive scan modes if you want to enjoy good-looking full resolution images on a computer screen. Canon has generally stayed ahead of the pack when it comes to offering progressive scan support.
The HG21's 24p Cinema Mode is what you would use if you want your video to have a movie theatre-like feel, and in many instances, we think that 24p is enough for general use. But it's a good thing that the Canon Vixia HG21 has 30p support.
One feature that Canon promotes with the Canon Vixia HG21 (and rightly so) is its support of a 24Mbps bit rate. This is the highest bandwidth available for an AVCHD camcorder, though it's not supported by all camcorders.
Why should you care about the bit rate? The imaging sensor in the Canon Vixia HG21 captures data at nearly 1Gbps. To get that data squeezed onto the little hard drive in the camera, the video needs to be compressed. The more compression used on a video, the more the image quality is compromised - you'll have video with blockiness in areas of low contrast. The higher bandwidth means that less compression needs to be used on the video, which makes your video of little Susie smoother and more detailed as she's running across the playground.
The Canon Vixia HG21 also shoots stills, but don't leave your still camera behind just yet. The 1,920-by-1,080 still images the camera produces are fine in a pinch, but won't be anything you would generally want to print.
Because it has a 120GB hard drive to store footage, the Canon Vixia HG21 is a little heavier than consumer HD camcorders that use only flash memory cards. You can store over 11 hours of video at the highest quality setting.
The Canon Vixia HG21 also has an SD card slot. This turns out to be more useful that it seems on the surface. Unfortunately, Canon forces you to connect the power adaptor to the HG21 in order to transfer video from the camcorder's hard drive to your PC; this can present a challenge if you are shooting on the road and a power outlet isn't nearby. Fortunately, there's a workaround: shoot to an SD card, then transfer video over using an SD reader.