In order to understand pushing/pulling film during development, let’s start with a little visualization exercise to illustrate the chemical process of developing film. Imagine a frame of film as an empty square. Drop a handful of sand onto the square--that represents the light that hit the film. In some places there is more sand, and in some places there is less sand. When you develop film, you are developing the areas on the square that were hit by sand. Two things determine the brightness/darkness of the image: how much sand hit the square, and how long it is in the developer. The areas that were hit by sand get brighter the longer they are in the developer.
When you push your film in processing, it is left in the developer for longer than normal.
This is usually done with underexposed film. But, this is where a lot of photogs get into trouble, with scans that have muddy/grainy shadow areas. Why? Let’s go back to our visualization exercise: if an area in the square was not hit by the sand at all, leaving it in the developer for longer won’t change the fact that it wasn’t hit by sand. Pushing the film might produce a small amount of detail in the areas that were lightly dusted with the sand (the shadows), but not by much.
This unequal rise in exposure—in which bright areas become lighter, while shadows stay more or less the same—increases the contrast and grain of the film. This is why pushing film in processing isn’t a way to “save” underexposed film as much as an artistic tool in your bag of photog tricks. When you are intending to push film in processing, make sure that you are metering for the medium to darker areas of the image in order to preserve details in the shadows.
When you pull your film in processing, it is left in the developer for less time than normal.
Pulling is used when film is overexposed and would benefit from less contrast. However, Richard does not recommend pulling film in processing, especially color film. Why? The development time is already quite short, and the process often results in a flat, murky look.
When pushing or pulling film in processing, it’s important to note that the most noticeable effects will be seen in the increase or decrease of contrast in the image, not an increase or decrease of effective exposure.