Quote Originally Posted by tkamiya View Post
I'm going to say this very simply ignoring all intricacies....

Push - treat the film as higher ISO than rated on box
Pull - treat the film as lower ISO than rated on box

Stop - relative unit measure of exposure. 1 stop means twice as much or half as much

So say you have a film rated at ISO 400. If you put it in your camera and set your ISO dial to 800, you are PUSHing. Further, you are pushing by ONE stop because your aperture/shutter speed combination will provide half as much light to the film.

Also, say you have a film rated at ISO 400. If you set your camera to ISO 200, you are PULLing. Further, you are pulling it by ONE stop because your film will get exposed twice as much.

Because your film will be over/under exposed, most of the time, development time gets adjusted. Typically more or less 20 to 25%. This is done to get the density/darkness of the film about right.

It gets infinitely complex from here but that's pretty much the basics.
It is very rare that I'll disagree with tkamiya, but ...

The "Pushing" part really refers to the development part of the process. As does the "Pulling" part.

If you want to increase the contrast of your negatives, you Push development.

If you want to decrease the contrast of your negatives, you Pull development.

When are you most frequently likely to want to increase the contrast of your negatives? When the light is so low you are forced to give the film less light than normal - i.e. under-expose it.

Most people accomplish that under-exposure by using their meter set at one, two or more stops higher. Than they choose the Push development that corresponds to the amount of stops they under-exposed the film.

A Push development doesn't really increase the density of the parts of the negative that correspond to the darkest parts of the scene. What it does is increase the contrast of those parts of the negatives that are in the mid-tones or shadows that are close to the mid-tones. So they print better. At the same time, a Push development increases the contrast of the highlights, which can make them harder to print.