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Masejoer The difference gets worse than that when the fog is thicker.
As it should. Any local light source with a wavelength less than the size of fog particles (all IR and visible) will make the image worse. With fog, it isn't so much the built-in IR that is the issue. Any light scatter between the camera and the subject will hurt visibility - you just need to focus light on the subject, from a source near the subject and away from the camera's line of sight. In the sample frame, the distant light sources will help to a degree, as they aren't localized near the camera. You will get backlighting of a subject in the frame though - maybe too low of contrast to do much with anyway.
That said, my experiences show that an external, directed beam of light at an angle to the subject between 20 and 60 degrees away from the camera's view of said subject seems to work well at limiting the affect of scatter. Shorter than 20 degrees, you get scatter more similar to built-in IR, and greater than 60 degrees, you begin to cast heavy shadows.
For simplicity in an average home, the easiest way to fight fog without causing excessive light pollution would be some higher output landscaping lighting scattered about, and no light sources near the camera. Directed at the ground, you will limit scatter while still getting sufficient fill lighting for video capture.
Fog isn't really an issue at our place, but I'll try to grab some frames of 3ft, 6ft, and 10ft height lighting when we next see fog. It could be good data for the forum. Probably December or January...
On topic - I really want to try out a couple of these BeSafe cams, but I really don't need more cameras. I still have four more waiting to be installed. I could see camouflaging some around the property just for the hell of it. I haven't dealt with longer CAT6 lengths - many could be around 250ft if I do.