In video investigations, the original tape should always be given to investigators. Video is then transferred to a computer in lossless format. A one to one transfer with no compression is needed because the other side will often need the same evidence to try to disprove your case. Sometimes they may even be using the same forensic software!
The original tape should be kept in some type of evidence locker to prevent contamination of evidence - you don't want the other side to accuse you of changing or purposely erasing the tapes. A record of everyone who touches the tape must be kept - this "chain of custody" ensures that the evidence remains unchanged from the original.
Chris Enzler from Cognitech says, "you won't get a perfect picture, that's Hollywood. You will get a good picture, but you can only reconstruct so much. You almost never get a nice tape - some videotapes have been recorded over 100 times. Banks and other stores try to be cheap, and too many people expect stuff from the movies or CSI."
Chris gave several examples of Hollywood magic that doesn't work in real life. One example is from a recent CSI episode, where the video investigator rotated a car in 3D to read the license plate, from a 2D video. In other shows you see the investigators enhance a single pixel to a full screen, with perfect clarity, which is obviously impossible.
"Most people expect the magic button, and a program that automatically knows what is wrong with the video. That doesn't exist."