Often when long exposures are discussed, there is a recommendation to use mirror lockup to avoid loss of image sharpness.
I fail to understand why the brief vibration, maybe half a second, could possibly affect a 30 second exposure? At the end of the exposure the mirror probably start to move after exposure end so this is not a problem.
Camera vibration caused by mirror slap should affect short exposures more than long exposures as I understand it.
What is the error in my reasoning?
Answer
The reason people recommend mirror lockup for exposures lasting several seconds is usually because they don't know any better.
Mirror lockup is most effective when the shutter speed is in the range of about 1/100 second down to around one second. Any shorter and the second curtain is closed before the vibration from the mirror reaches the parts that count: the lens and the sensor. Any longer and the duration of the vibration significant to create blur more than one pixel wide is such a small percentage of the total exposure time as to be trivial. If a person can spend three or four seconds to walk across the field of view of a 30 second exposure and not show up in the resulting image then the result of mirror vibration on a 30 second image is likely to not even be detectable.
There is one exception where mirror lockup can be helpful with exposures longer than about one second. If you are shooting in a very dark environment and there are very bright light sources included in the frame, that first second of vibration can cause noticeable light trails, even when the total exposure is 30 seconds or longer.
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