Benjamin Marks
Veteran
At the extremes, it is helpful to review what film is and what fixer does. Film is, generally, a purified animal gelatin infused with silver salts on a polyester base. Fixer (sodium or ammonium thiosulfate) at working strength is a weakly acid solution which converts unexposed silver halides to a water soluble form so that they may be washed out of the gelatin in film or paper. If left in contact with your film or paper long enough, they can eat away at the silver in the emulsion and/or damage the base (more likely with fiber paper than film, IMHO). Then again, I have had the gelatin slough off of film base that was left in water too long, so there are always risks going to extremes with this. Once all of the unexposed silver halides have been converted, there is no more work for the fixer to do. Fixing after this point is likely only have negative effects. More importantly from a processing perspective ("But mom, I intended to end up with a thinner negative") is the repeatability of this process. If you are fixing for a random amount of time in part of your process, it will be much harder to adjust when problems arise and you are trying to pinpoint a cause. As a practical matter it is difficult to exhaust film fixer in a single session unless you are really developing a lot of square feet of film. Stick to the manufacturer's recommendation and you should be fine.
Ben Marks
Ben Marks
