Having not been working much recently due to Coronavirus, I have had a lot more time for photography. Apart from testing cameras and trying to get some new prints I have also been reading and processing (mentally and in the darkroom). Having been using various cameras for 30+ years I have generally used mirror lock up to minimize vibrations on longer exposures and especially with medium format cameras. MLU has always been second nature. If exposure times are getting longish, use MLU to eliminate that vibration when the shutter fires, but,,,,Thinking about it recently, and possibly thinking about it too much and confusing myself, I now have a question that may be better in the beginner forum. MLU is something I have used forever but cant for the life of me now see why only at long exposures. When we make an exposure, the mirror lifts up which may cause some vibration and thus negatives that are not as sharp as they could be. Lets say the mirror vibration lasts for 1/4 of a second (complete guess on my part as to the duration) and the exposure is 4 seconds. Disregarding the delay between the mirror going up and the exposure starting, the mirror vibration would then last 1/16th of the exposure time. With an exposure of many minutes, the vibration would only last a tiny fraction of the exposure time and may not be recorded on the negative at all. If the exposure was 1/4 second or faster, then the entire exposure would be made during the vibrations. Why then is MLU used on longer/slower exposures, when the vibration could be present on only a small part of the shutter open time, and generally not on shorter/faster exposures, when the vibration could be present for the entire exposure time?