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Before the days of digital film workflows where digital visual effects could be applied in non-linear editing software and other programs all effects had to be done at the lab directly on the film stock. This is still done today, though it is often not done on smaller projects. These differences at the lab do not always go as far as being obvious visual effects, but are often just choices in how they want the film to be processed. This could be a response to mistakes in production, special circumstances, or just visual choices bordering on visual effects. One of the most persistent of these processing choices is call push processing.
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Using Push Processing
Push processing, which is also referred to as "pushing the film," is a base type of overdevelopment. What happens here is that the film stock itself is submerged in the solution used for development longer than normal. The silver flakes that are used to be developed in the film develop more than they normally would be. This makes the overall image have a more palatable sense of density. This push processing is often used to try and deal with a film stock that was underexposed.
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Film Stock Exposure
Since push processing can compensate for underexposure it is no surprise that it is often described and communicated in terms of f stops. For example, a single stop of push processing will half the amount of required light to pick up the image during filming. This is, of course, relative to the speed of the film stock that you are using. This means that if you are using a fast stock you would be able to film in almost blackness with a single stop of push processing. Slow film stock would be even more responsive to push processing and this would be incredibly useful.
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Extra Push Processing
If you are using push processing you are going to notice that there is some interference in the image quality. Over all you will get an increase in general contrast and grain in the film image. The color will be less dramatic and appear much more desaturated after push processing. This is why you usually do not want to go much more than a single stop in push processing. If you do go two stops or more you are going to see a very dramatic alteration to the image. This can be done if you are trying to apply a type of visual effect to the image, but it is not going to look natural and will look very washed out. This can be a great choice to get a gritty and otherworld visual effect. If you are going to try and use push processing for visual effects on your film stock then you should try to do a camera test of a few test rolls of film.