I’ve tried to keep this post brief and basic, and just explain what the difference between these two terms is. I’m going to follow it up with a couple of posts explaing why you might choose one of the other of these for colour or B&W film. They should be linked below once I write them.
A lot of people new to film get confused at the difference between overexposure/underexposure and pushing/pulling film. This is understandable, because the terms refer to very similar things, with one main difference. I’m also going to use the term “box speed” here:
Shooting at Box Speed means that you use the official manufacturer ISO for the film (ie. the ISO on the box).
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.
So basically, 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 box speed. If you push or pull however, your photos should look mostly the same as shooting at box speed, but the amount of contrast and grain will be a bit different (I’ll explain that later).
Probably the most time that you hear about pushing or pulling is when people talk about pushing Black & White film. I’m going to explain a typical situation where you might do this:
Pushing B&W film: An example
Imagine you are out with your camera one evening, 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 box speed (which is ISO 400 for HP5), your shutter speed will be 1/15 seconds.
“Oh no!” you think, “I know from experience I can’t hold my camera steady for a fifteenth of a second. My photos will all be blurry.”
Pushing your film is the solution to this situation! 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 box 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. Finally, some cameras have neither of these options, and just read the film’s ISO directly from the DX Code on the canister (a simple bar code), in which case you need to learn about DX Code hacking (a whole other topic).
So now, once you’ve tricked your camera into thinking you’re shooting ISO 1600 film, you go about your business and shoot the film. Once you’re done, you make a note that you have pushed the film 2 stops, and now it’s developing time.
Most labs that develop Black & White film will do the next part for you. You just have to tell them “Please push process this, I shot it at 2 stops above box speed, ISO 1600”, or something like that – it’s probably extra smart to marker this onto the actual film canister too.
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, so just consult that.
Once your film is developed, you will now magically have a bunch of photos that look like they were exposed at box speed, even though you didn’t have enough light to do so.
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.
Pulling B&W Film
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 (whether you or the lab is developing it), and your photos will have less contrast and grain.
In practice, pushing B&W film is much more common than pulling.
What about colour film?
Pushing or pulling colour film is entirely possible, but not as common either. One reason may be that 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 labs don’t really stick these in big machines, they 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.
One last, important thing to remember
If you choose to push or pull a roll of film, you have to commit for the entire roll. That’s because you’re going to be putting the entire roll in the developing chemicals for a longer or shorter time.
There are a number of stylistic reasons that you might want to underexpose or overexpose a photo, without compensating for it later. If that’s the case, you don’t need to deal with pushing or pulling the film.
I hope this explains the difference between these 2 things decently, and I guess I sort of veered into explaining the whole concept of pushing and pulling. I still plan to write a couple of posts about doing this with colour film though, so if they’re not linked below, they should be soon!