Originally posted by thamwk
This is how it works. Basically you re-rate your film(print or slide) to another setting recommended by the manufacturer and exposed it.
For example, you can re-rate Provia 400F at ISO 800 or ISO 1600. When you bring the exposed roll to the lab, you have to tell them to push process the roll. It it is rated at ISO 800, then asked them to push 1 stop. If rated at ISO 1600, ask them to push 2 stop.
What the lab will do then is to proess the film for a longer period of time. This take up thier machine time and need more chemicals (this part not very sure). Thus you have to pay more for push processing, even more for 2 stops.
So far for slide films, only RGB is able to push 2 stops, K T Photo at Tiong Bahru only able to push 1 stop.
Generally, it is more grainny for push processing.
Each type of films can be pushed process, look out for the specifications on Kodak or Fuji's web sites.