Every image of waterfall is taken at long, sometime very long exposure. sometimes I wish I could see it as nature offers: no long exposure. My question is: Is there any way to work back from the long exposure?
You should take the shots at many different times exposures. Then pick the one that gives you the best effect of the running water, or possibly combine several shots to get a result better than the shots that you took. I don't know how you could get a good result any other way. No technique in POST that I know of is capable of what you are asking.
