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"Push" & "Pull" Processing


This procedure requires an adjustable SLR camera.

What is push - pull processing?

This is a technique that can be used in the darkroom to produce acceptable negatives from black and white films that have been exposed at the wrong film speed (ISO or ASA). In other words, you have intentionally or accidentally set your cameras ISO setting to the wrong value. This will not change the light sensitivity of the film, what you have done is to either underexpose or overexpose the film. This can be corrected later in the darkroom when developing the film, by either over processing or under processing.

  • Push Processing

    Giving the film more development time to compensate for underexposure in the camera.

    Example:

    You exposed a roll of ISO 100 film at a setting of ISO 400 - 2 stops difference.

  • Pull Processing

    Giving the film less development time to compensate for overexposure in the camera.
    Use pull processing to compensate for NO MORE THAN one stop of overexposure.

    Example:

    You exposed a roll of ISO 400 film at a setting of ISO 200 - 1 stop difference.

Push or pull processing affects your negative's contrast, so you must decide whether adjusting the print exposure time, or the film processing time is best.


When would this be useful? -

  • Case #1

    You are out shooting pictures and have just realized that you have accidentally set your ASA to the wrong value.

  • Case #2

    You are out shooting pictures outdoors and then unexpectedly find that you have to shoot indoors, in a low light situation without the availablity of a flash. You only have ISO 100 film with you and you really need to get the shot.

How do I do it? -

  • Case #1

    If you have discovered that you have set your camera to the wrong film speed setting (ISO), leave the camera at that setting until the whole roll of film has been exposed. You can then use the appropriate technique - either push or pull processing, which will allow you to salvage your pictures.

    If you have set the ISO to a setting lower than the rated film speed, then you have overexposed the film and will need to develop it for less than the standard time that is recommended by the manufacturer. This is a "pull" process.

    If you have set the ISO to a setting higher than the rated film speed, then you have underexposed the film and will need to develop it for longer than the standard time that is recommended by the manufacturer. This is a "push" process.

  • Case #2

    You are indoors or in a darkened area, with only slow speed film available. Change the 100 ISO film that you currently have in the camera, to a fresh roll and intentionally expose this entire roll at the wrong ISO. Raising the film speed setting to ISO 400 will allow you to attain useable shutter speeds (to avoid blur) and to obtain a greater depth of field. This setting will give all of the negatives on the roll a 2 stop underexposure.
    This is called "pushing" the film and it will have to be developed for longer than the manufactures recommended time for a normal ISO setting. This can be developed at a commercial lab, or in your home darkroom.
    One other point...expose for the shadows, because this will ensure that you will print good shadow detail, and it will allow you to burn in the washed out highlights .

What are the effects?

An increase in graininess, and poorer quality tonal reproduction in the final print, but these may be acceptable if you absolutely have to get the shot. In fact you may want in some circumstances to attain a fairly grainy image for effect, so if that's the case just push by 2 or 3 stops and process accordingly.


How many stops is it? -

Two stops of over or underexposure will usually require processing changes.

100 ISO film with camera set at 200 ISO is a 1 stop underexposure
(1 stop over / underexposure may be printable with normal development)

100 ISO film with camera set at 400 ISO is a 2 stops underexposure
100 ISO film with camera set at 800 ISO is a 3 stops underexposure

400 ISO film with camera set at 800 ISO is a 1 stop underexposure
(1 stop over / underexposure may be printable with normal development)

400 ISO film with camera set at 1600 ISO is a 2 stops underexposure
400 ISO film with camera set at 3200 ISO is a 3 stops underexposure


400 ISO film with camera set at 200 ISO is a 1 stop overexposure
(1 stop over / underexposure may be printable with normal development)

400 ISO film with camera set at 100 ISO is 2 stops overexposure


How do I process the film?

Processing times for "push" and "pull" processing, should be considered as starting points only!

See Film Data Tables for processing times of an assortment of common film types, when exposed at normal film speeds and at other than normal film speeds.

Kodak's rule of thumb for push processing, is to increase development time by two minutes for each camera stop of underexposure.

For pull processing, decrease the development time one minute for each stop of overexposure.


What about using this technique with color film?

Black and white negative films and color reversal films (slides) can be push processed. Color print films however have a far greater latitude than do color slide films. A color print film can be incorrectly exposed by as much as 2 stops and still produce a satisfactory print using standard processing. Also when color reversal films are pushed, they may develop strange color casts which are difficult to correct. For these reasons, do not attempt this technique with color reversal films.



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