I have found, that if it is underexposed too much, it scans terribly. Looks okay on a light table, but it does not scan so well. Which also throws the scanner focus out (I have calibrated my scanner a V850). When the slide/neg is exposed more or less properly, they scan well with good detail.

Any rules of thumb for adjusting exposure from what is metered? I am guessing at least two stops brighter if the scene is strongly back lit or very dim to start.