Before getting into push and pull processing, it helps to understand box speed.
Box speed
Box speed is the ISO rating printed on the film box by the manufacturer. It tells you the speed at which the film is designed to be shot and developed for its intended look. For example, if a roll is rated at ISO 400, shooting it at 400 and processing it normally means you are using it at box speed.
This is usually the best place to start, especially for beginners, because it gives you the most reliable idea of how the film is meant to perform in terms of exposure, grain, contrast, and colour.
Push processing
Pushing means rating the film at a higher ISO than its box speed and then adjusting development to compensate. For example, a 400 ISO film might be shot at 800 or 1600 and then developed for longer.
Photographers usually push film when they need to work in lower light or want a grittier, more contrast-heavy look. The trade-off is that pushed film often shows more grain, stronger contrast, and sometimes less shadow detail.
Pull processing
Pulling means rating the film at a lower ISO than its box speed and then developing it accordingly. For example, a 400 ISO film might be shot at 200 and processed to match that exposure.
This is often done when a photographer wants softer contrast, more controlled highlights, or a smoother tonal range. Pulled film can appear gentler and less harsh, especially in bright conditions.
A simple way to think about it
Shooting at box speed means using the film as intended.
Pushing helps when you need more speed or want a rougher, more dramatic result.
Pulling helps when you want a softer and more relaxed look.
For anyone starting out, it is best to first understand how a film behaves at box speed before experimenting with pushing or pulling.