Defined: Pushing/Pulling Film VS Under/Overexposure
One great thing about photographic film is the variety of ways you can use it. People new to film may be unaware of these so I wanted to explain a few terms that come up. Pushing/Pulling Film, Under/Overexposure specifically.
Underexposing film means that you change your settings so that less light than recommended hits the film.
Overexposing film means that you let more light than recommended hit the film.
Pushing film means that you underexpose it, but also develop it for a longer time, to compensate for the underexposure.
Pulling film means that you overexpose it, but also develop it for a shorter time to compensate.
When you use the terms pushing and pulling, the key difference is that the actual development process is different than normal.
When you overexpose, or underexpose film, but don’t compensate for it while developing, you’ll get photos that look brighter, or darker than if you shot at normal speed. If you push or pull however, your photos should look mostly the same as shooting at recommended speed, but the amount of contrast and grain will be a bit different. Your images will have more contrast when you push the film, and will also be more grainy. Some people will push film just to get this specific effect.
Pulling film, as you may have guessed, works basically in the same way, except you’re adjusting everything in the opposite direction. So your camera will take longer exposures, the film will spend less time in the developer. Your photos will have less contrast and grain.
If you choose to push or pull a roll of film, you have to commit for the entire roll because you’re going to be putting the entire roll in the developing chemicals for a longer or shorter time.
Pushing B&W Film
Imagine you are out with your camera one night and you have a roll of Ilford HP5 and you want to take some photos. You check your camera’s meter, and it says that if you shoot at the required speed (which is ISO 400 for HP5), your shutter speed will be 1/15 seconds. If you're concerned that your pictures will turn out blurry you can push the film, all you have to do is to trick your camera into thinking your roll of film is a higher ISO, perhaps ISO 1600. This is 2 stops higher than the recommended speed, so you can use a much faster shutter speed. For each stop, you double your shutter speed, so you can now take pictures at 1/60s (four times faster than 1/15s).
A common way to trick your camera is to set the ISO dial to 1600. If your camera doesn’t have an ISO dial but does have an exposure compensation setting (which is often marked as EV+/-), you can set that to -2.00. Make sure to take note that you have pushed the film 2 stops, perhaps by writing it on your film canister. This will be useful later when you go to develop your black & white film. You can take it to a film lab and let them know that you shot it at a higher speed or develop the film yourself.
If you develop film yourself, you will just need to leave the film in the developer solution for longer than normal. How much longer depends on how many stops you pushed the film by, and what film and developer you’re using. The Massive Dev Chart is the ultimate reference for this.
There are a couple of trade-offs however. Your images will have more contrast when you push the film, and will also be more grainy. This is not necessarily a bad thing, depending on what you’re looking for, and some people will push film just to get this effect.
Colour Film
Pushing or pulling colour film is entirely possible, but not as common for colour film. Photo labs tend to develop colour film in large, automated machines, and they don’t have the flexibility to leave some film in the machine for longer than everything else. This isn’t the case with B&W film, because all the different types already have different developing times, so photo labs tend to use a more manual process.
If you develop C-41 colour film yourself though, you can certainly push or pull it, and the instructions that come with your home developing kit should cover that.
