It's important to look at the cameras specs. There are some sellers out there that are selling cameras that are advertised as being HD but are only SD. So it pays to be careful when buying a camera.
HD (High Definition) is much more preferable to SD (Standard Definition). When using a HD camera videos are clearer and sharper than SD and when you are trying to capture number plates, the more detailed the better.
Things to look for are video format. High Definition will typically have a movie format such as .MOV, H.264 compression. Resolution will be something like 1080p = 1920—1080 pixels (16:9), 30 fps, 960p = 1280—960 pixels (4:3), 30 fps, 720p = 1280—720 pixels (16:9), 60 fps, 720p = 1280—720 pixels (16:9), 30 fps.
Standard Definition will typically have a video format that is either AVI, or MJPEG. The Resolution is something like, 640 x 480 or 720 x 480 and some maybe up scaled from 640 x 480 to 1280x960.
Try to avoid cameras that use M-Jpeg video format.
Motion Jpeg is one of the oldest video formats, unlike MPEG or H.264 formats, M-JPEG takes a very different approach to video compression. Instead of compressing the entire video as a whole, each frame is compressed as an independent JPEG image, and strung together to make a video sequence.
The downside of Motion JPEG is, video playback can also be quite jerky and it does not use any compression, because they are already Jpeg images, so the resulting video files can be quite large. A H.264 encoder can, reduce the size of a video file by more than 80%, compared with the Motion JPEG format and as much as 50% more than MPEG-4, without any loss of image quality. JPEG also has a problem with sharp edges which is unavoidable, partly because the standard was meant to compress natural images like scanned photographs.
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Lens Viewing Angle
Some cheap cameras will have a viewing angle of approx 70º, this is pretty narrow. While others such as the GoPro have a viewing angle of 170º ultra wide angle in 720p and 127º wide angle in 1080p.
The advantage of wide angle is that it allows for a lot more action to be captured on camera. Depending on what you use the camera for, 70º maybe to narrow, while a viewing angle of 170º tends to give a Fish Eye affect and some people don't like this, but it captures the most action. 127º is the probably the best all round viewing angle.
Another thing to keep in mind, is the narrower the field of view, the harder it is to line the camera up properly. The cheaper cameras do not have a built in screen, so it can take a few tries before you find the correct recording position. The mid to high end cameras usually come with either a screen or laser, this makes lining up a lot easier. If the camera has a viewing angle of 170º all you need to do is face the camera forward and chances are it will be capturing what you want. Although after viewing the footage you may need to alter your mount to get it cantered properly.
Here is an animation I made to show you the different viewing angles.
Graeme