Here is a reply that I wrote on another post here today that goes into the "why" of why we give some maximum resolution based on the sensor size.
Most cameras will look great during the day. It's at night/low light situations where cameras will start to give less than stellar results. At night, if the camera is set for auto exposure, then the camera will slow the shutter speed down and crank the gain up until it produces a very nice bright image. These images look great - until there is something moving in the frame. Then the slow shutter speeds and high gain will result in blurry and ghosting subject (ghosting is where parts, or even all of the moving subject will disappear in some frames of the recorded footage). A PTZ with autotracking can have a slightly slower shutter speed than a fixed camera and still produce decent images because it will track the subject which counteracts the slow shutter speed blurring the motion, but there is still a limit before the subjects will by blurry.
We are judging these things on physics. You camera has a known number of pixels (5mp) and a known image sensor size (1/2.8" which is roughly 1/3"). Experience has told us that the pixel size that these two things create is not going to give great low/no light images. Of course if you have your camera mounted in a bright area (perhaps there are some bright street lights nearby), then it may produce great results. But the vast majority of people are going to have a tough time getting the gain low enough to prevent ghosting, the shutter speed high enough to prevent blurring, and still be able to get an image that is bright enough to be useful.
A camera with only 2mp that uses the same size sensor (1/2.8") will have pixels that are more than twice as large as the pixels in your camera. This means each pixel is able to absorb more than twice as much light in the same amount of time as your camera. This results in much brighter images and it is generally the highest resolution (2mp) that we suggest on a 1/2.8" sensor size to ensure that we can set the gain low enough to prevent ghosting, set the shutter speed fast enough to prevent blurring, and still have a bright enough image to be useful. Any higher pixel count and the image will be too dark after setting appropriate shutter and gain settings.
So what are you to do? Well as I already mention, this is highly dependent on the amount of available light in your setting. The more light you have, the greater the chance that you can break our "maximum pixel rule" and still get usable images. Outside of adding more available light, you need to put your camera in manual exposure mode and play around with the settings (gain, aperture/iris, and shutter) to get the best compromise. You might have to live with some blurring and some ghosting in order to get an image bright enough to be useful.
Hopefully that helps. We aren't trying to crush your excitement about your new camera, but we do want you to look at the performance of it with open and honest eyes. You are going to have to play around with the shutter, gain, and aperture/iris settings with a moving subject at night to find the right combination of settings to give you the best result. If you leave the camera set on auto, it will look great with static images, but as soon as you have a subject moving in the frame, you are going to have really poor results (ie unusable footage to identify a subject). By finding the right combination of settings for your location, you will maximize the chance that you will get good identifiable footage of a moving subject at night.
I hope that helps explain it a little further. We are here to help, so don't be afraid to ask questions or even ask for help dialing in your settings.
