Pushing and pulling of films

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Can anyone enlighten me about the pushing and pulling of B&W films? The different effects and perhaps some recommendations. And also how about the developing time of the film?

-- Anonymous, July 21, 1997


RE: Pushing and pulling of films

Dear Jerry,

many photographic publications, magazines and even the majority of film manufacturers insist on push and pull film processing as a mean to alter film sensitivity.

The basic idea is as follows: - you can underexpose a film and - provided that you allow extended developing time - achieve correct results, as if you were using a faster than nominal film.

Behind the pull processing there is the same basic idea (reversed): - you can overexpose a film and - provided that you allow reduced developing time - achieve correct results, as if you wre using a slower than nominal film.

The reality is slightly different from these assumptions. If fact, what actually changes varying the developing time is film contrast, not the speed.

So, if you expose a 400 ASA film for a nominal sensitivity of 3200 ASA (3 stop underexposure) and develop accordingly, you will obtain a film where the subject is clearly distinguishable, but without any detail in the shadows and with an overall contrast very high. This is because the shadow sensitivity isn't significantly changed by the extended developing time.

The second side-effect of push-processing is the increase in maximum negative density, that often causes a lot of problems in printing the image, requiring extensive "dodging" and "burning".

The third side-effect of push-processing is the increase of image granularity. This effect involves any kind of non-chromogenic B&W film, but with huge variations between different film types. If - for aesthetic and creative purpouses - you are in search for grainy images you can use push-processing, but you will have to test different materials to determine a film-developer combination suiting your needs.

The fourth side-effect of push-processing is the increase of film fog: extending the development time over reasonable limits means obtaining a negative with a substrate not so transparent...

It's up to you to judge if this kind of result fits your photographic needs or not. If you are a reporter and the push processing allows you to save shoots that would be otherwise lost the answer is probably yes. If you are a landscape photographer following Ansel Adams footsteps, the answer is quite probably no.

Pull-processing, on the other hand, shows the reverse of the same effects. Beware, this doesn't mean that overexposed and underdeveloped images are necessarily good images.

Whilst better technical result can in general be obtained with a slight overexposure and consequent slight underdevelopment, remember that an excess in underdevelopment brings to negatives that are too transparent, and they are difficult to print and tend to show more apparent grain than normally-developed negatives. The images obtained with strong underdevelopment are VERY VERY VERY soft (low contrast) and look like they were taken through a "flou" filter. They cannot be corretcly printed even using high contrast paper.

In the Zone System extended and reduced developing times are used to adjust film contrast basing upon subject contrast. This is - in my opinion - the only "correct" use of push and pull processing, allowing the achievement of splendid results. Note, however, that pull processing is usually limited to 1 stop and push processing to 2 stop (for extremely low-contrast subjects). Extending development variations above this limit produces "unacceptable" evidence of the effects listed above.

Obviously, I used quotation marks to remember you that - in photography - the concept of "correctness" and the validity of the results are purely personal and depend upon the photographer's aesthetic purpouse.

Briefly, this is what I know, what I think and what I experienced about push and pull film processing. If you have deeper interest in these themes, don't be afraid to read sensitometry manuals. My suggestion is to read "The negative" by Ansel Adams that - by the way - is the bible of "zone system". You can freely use my e-mail for further clarifications.

May the force be with you


-- Anonymous, July 23, 1997

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