Pushing and Pulling Film
The pushing and pulling of film only refers to the development process. It can only be applied to the whole roll of film.
The film in the camera should only be considered as overexposed or underexposed. It should not be referred to as being pushed or pulled.
Using this terminology simplifies the understanding of pushing and pulling film. If you overexpose a film in camera then the exposure may be pulled back at development time. If you underexpose a film in camera then the exposure may be pushed up at development time.
When film is pushed during development, then either the development time is increase or the temperature is increased or both. This has the effect of increasing the density of the negative. This is usually done to compensate for underexposure in the camera.
When film is pulled during development, then either the development time is decreased or the temperature is decreased or both. This has the effect of decreasing the density of the negative. This is usually done to compensate for overexposure in the camera.
Why Pull or Push?
There are two main reasons for pulling and pushing - artistic expression and control of shutter speed.
Pushing - The film will be underexposed on purpose so that it can be pushed at development time. Pushing means a longer development time and increased contrast. Pushing film will also give you more grain. Pushing will increase the density in the midtones and highlights and increase saturation in colour films .
Pulling - The film will be overexposed on purpose so that it can be pulled at development time. Pulling film means a shorter development time which will result in reduced contrast in B/W film and reduced saturation in colour film. Pulling brings out details in the shadows.
Shutter Speed Control
In poor light, when shooting at film box speed (ISO), the shutter speed might be too low to freeze moving subjects. By shooting at a higher shutter speed, action can be frozen and effects of camera shake reduced. However, the film will be underexposed. This underexposure can be compensated somewhat by pushing the film at development time.
Automatic Exposure Cameras
Remember, pulling and pushing is applied to the whole film. If your camera is able to auto-expose based on the ISO value of the film, then you can consistantly over or under expose by changing the film box speed to a different speed. If you half the ISO setting in camera then you will consistantly overexpose by 1 stop. Ask the film developer to pull by 1 stop. If you double the camera ISO setting you will consistantly underexpose by 1 stop. Ask the film developer to push by 1 stop. Similarly, If you set the ISO to a quarter of the box speed then you will consistantly overexpose by 2 stops. Ask the film developer to pull by 2 stops. If you quadruple the camera ISO setting you will consistantly underexpose by 2 stops. Ask the film developer to push by 2 stops. See the Push/Pull Calculator below.
Manual Exposure Cameras
Again, pulling and pushing must be applied to the whole film. Set yout exposure meter to a different ISO to the film box speed. Continue as above for automatic exposure cameras.
Cameras with limited exposure control
Many vintage cameras have limited or no exposure control. In these cases, pulling and pushing is not really applicable. However, it may have its uses. Lets say you have loaded your box camera (shutter speed 1/50s and aperture f/16) with ISO 400 film and you take a whole roll during a sunny summers day. Using the sunny 16 rule, you will see that your film will be overexposed by 3 stops. Negative film can usually deal with this, even when developed normally and especially if the negative is scanned and manipulated digitally. However you may decide to pull this film by up to 3 stops at the time of development. This would give a less contrasty black & white image or more muted colours. It would also bring out detail in the shadows.
Calculate how much you need to pull or push film from box speed at the time of development. Pushing more than 2 stops or pulling more than 3 stops is not recommended.
(* Not Recommended)