I took this pic with OM-D E-M5 with a 12mm F2, at F2, 25 seconds, ISO 4000. Temperature is about -3 Celcius. Is lens fogging up or is it a sensor problem? There didn't appear to be any fog on lens when I brought it inside. The problem got worse the longer I stayed outside. Initially it was just noticeable at top left edge, and gradually moved down. Any ideas? (First time taking long exposures at night in cool weather). I took pics all day outside with same and other lens without problems. Subsequent pictures inside didn't have any problem.