How do traffic speed cameras focus on a lisence plate to take a readable pic?

I live and work in China where they now using cameras to detect speed. This is very effective as the police here tend to do nothing at all, so automated devices are the only means to catch speeding drivers who tend to drive poorly at any speed. I was told that the lisence plates have a focal point, which I do see on my plate. It is a recessed dimple about one quarter inch in diameter and it appears to be a different alloy, but it may be my imagination. It is NOT, painted for some reason, and most are rusty. There is definately some reason for the dimple and the fact that it is NOT painted. Many people here completely paint over the entire plate to hide the white numbers from the camera. I am not sure if it works, but the local police do not seem bothered by the fact that 30% of the cars have lisence plates with hidden numerals. Just curious how these cameras focus and what the technology is regarding the dimple in the plate.How do traffic speed cameras focus on a lisence plate to take a readable pic?
%26lt;%26gt;The cameras are pre-positioned to optimize the picture of the car. High-speed digital cameras make it easy to identify the speeder. There is no need to focus on a particular object. Sensors in the roadway trip the cameras when a vehicle passes. Here is a description of one particular system:

http://www.rta.nsw.gov.au/roadsafety/spe鈥?/a>