If you shoot a roll of film all in the same light (for example you shot a roll on a cloudy day) you can "push" the film to increase the contrast. If you shot a whole roll on a bright sunny day, you may want to "pull" the development. A simple way to do a push or pull is to add or subtract a minute (or two) from the development time.

Adding a minute will make the light areas of the film lighter and increase the contrast. Pulling the film, by decreasing the development time, will decrease the film contrast. A typical reason to pull a film is night photography. You should give the film plenty of exposure to begin with, and now the bright areas (for example window lights from a building) would be too bright to print with any detail. By pulling the film, the light areas are less bright and it should be easier to print.

The proper way to do a pull is to change the Exposure Index of the film initially. Setting the ISO on the camera meter to 200 when you are shooting 400 ISO film will cause extra exposure. Pulling the film one stop (about a minute less) will keep the bright areas in the print from going all white and maintain some detail.

To push a 400 ISO film, initially set the exposure meter to 800. You can now increase the contrast in development. Pushing a film is often used when shooting in low light.

ISO, Stops, Exposure and Exposure Index are explained in detail in the MOPC program.