Reasonable doubts

According to Simon, Baltimore City juries are notorious for letting people off. All doubts are reasonable. It is a question I hadn't thought of before. What is a "reasonable doubt". Whenever I hear about a person who is exonerated for a crime for which they have already served time, or received the death pentalty for, I wonder where the line for reasonable doubt can be drawn. Is it worse to have a guilty person go free, or an innocent person punished? I am reminded of a movie, interestingly enough with a Baltimore connection: ...And Justice for All. I must have been 15 years old when I saw it the first time. I remember after it was over saying to my sister "Hey, all the innocent people are in jail, and all the guilty people went free." To which she gave the response "Duh. It's a satire."