>>3183999When they create your print, they use a combination of color filters and lights to print from the neg. If you don't specify something, it's usually a "best light" grade for the whole roll, which is an average from beginning to end. So if you have mixed over/under exposures on the roll, they won't be corrected properly.
Depending on the scan and codec, you can do quite a bit actually, especially if it's a negative scan.
Don't worry about the purists, if grading on the computer makes your footage look better, then do it.
