Any photographic film has several parameters. Film format - 35 mm, 120 or 110; type - color or black and white, number of frames and light sensitivity. While you can't influence the first three parameters, the ISO of a film is a quality you can adjust!
You've probably heard the expressions "push film" and "pull film". Simply put, it means the following: pull is to decrease the sensitivity of the film, and push is to increase the sensitivity of the film.
There are situations in photography where the sensitivity of the film you originally chose is not quite right for the scene. For example, you have ISO 400 film and you have to shoot not just in low light conditions, but in very low light conditions. You could increase the shutter speed, but it's not always possible to do it with your hands, and not many people carry a tripod with them; and the full aperture is not suitable for the situation or it's not enough. What do you do in that case? You could shoot the film as if it had a higher ISO: 800, for instance. Then develop it at home or in the lab as an 800 film. This process is called Push.
The opposite story. You have an ISO of 100, but you want 50 because the sun is too bright, for example. Then you shoot the film as if it had a sensitivity of 50 and then process it with the same value in mind. This process is called Pull.
Essentially, we are dealing with exposing the film a few stops up or down. One stop is equal to doubling or halving the ISO. That is: if your film is ISO 400, then one stop pulling process makes it into ISO 800, two stops makes it into ISO 1600 and so on. When you turn the film in to the lab, be sure to indicate to the operator on the reel how many stops the exposure was lowered or raised (+1 or -1, respectively.). If you don't, the film will be developed at face value, and therefore the result will not be what you expect. For the lab, these marks are of great importance, since push/pull film development times are different. Push takes longer to develop, while pull takes less time. The cost of processing such a film also increases.
One more important point: if you decided to shoot the film off-rated, you should do it with ALL the film reel. Otherwise, some frames will be fine, and others will be underexposed or overexposed.
Why push/pull at all?
Here are a few reasons:
1. Push is needed most often when you're taking photos in low light: when you can't get the right exposure using other methods. This includes taking photos when it's cloudy, for example, or when there's little light, and handheld photography (without a prop/tripod) is not possible. Push allows you to use shorter shutter speeds.
2. With Push, you can achieve interesting effects. This process increases contrast and grain becomes more visible. The picture becomes brighter. With color film, you can get saturated or even distorted colors.
3. Pull is useful for taking pictures in very bright conditions as it helps draw out the details in the shadows and will reduce the contrast.
4. Pull can also be used for effects. Colors with this process become less bright or even faded, reducing the overall contrast of the photo.
As you can see, the push and pull processes can be used for a variety of purposes - both "technical", to correct exposure when taking pictures, and for creative purposes, but you should always keep in mind the side effects described above.
What else do you need to know?
In theory, these processes are possible on any camera. But the best results come out, of course, if you use cameras with a good lens and accurate exposure metering. These are also cameras where you can manually set the ISO of the film before shooting. On a point-and-shoots, processes are possible if you initially change the ISO on the reel, since almost any compact reads the DX code on the film roll and exposes accordingly to the rating.
Films are different and not all lend themselves to push and pull, especially when it comes to overexposed or underexposed by 2 stops or more. It is important to understand that the best results are obtained with black and white films, as they are more amenable to processing and usually have a higher ISO range (dynamic range). Color can get very "blurry" in processing. Also, professional segment films are more tolerant of push/pull because they are better quality than amateur films. In fact, that is why they cost more.
Most often it is black and white films that are Push/Pull processed intentionally: to get high contrast and noticeable grain. Color is also processed using these processes. Slides are pushed or pulled very rarely, as this material is quite capricious and does not forgive exposure errors. The maximal push for a slide is 1-1.5 stops. Important note about expired films: if you shoot a slide at ISO 100 as 50, you don't need to do it, because you initially assume that the sensitivity of the film has decreased to 50. When you take such film to the lab, you're just giving it to the lab for normal development.
The best black and white films for these processes are Kodak TRI-X, Ilford HP5 Plus, Ilford PAN 3200. Color films are Kodak's Portra series (at ISO 400 will do best). Before you fluff or pultivate some film we advise you to look in search engine and see the results to have at least an approximate idea of what the result could be. All films are different, and there are some that are better not to process at all. However, nothing stops you from experimenting. Especially if you initially want to get some unusual result.