Introduction: what is Push or Pull processing?
All films have a stated ISO/speed, also known as ASA. This defines how sensitive to light the film is. For example, Ilford Delta 400 is an ISO 400 film. It’s twice as sensitive to light as an ISO 200 film, which, in turn, would be twice a sensitive to an ISO 100 film. This system has been carried over into digital photography and most digital cameras, including many smart phones, have the option to change the sensitivity of the sensor in ISO ratings.
Push or Pull processing is often required where a particular film has been exposed at a different ISO to the manufacturers stated speed on the box. The method involves changing the development time and deviating from normal. For example, shooting Ilford’s black and white film Delta 400 at ISO800. Under these circumstances it is usually preferable to “up-rate” the development in order to compensate for underexposure. Typically, in this example, you would “push 1 stop”, as it has been exposed 1 stop faster than the official speed. If the film was exposed at ISO200, then a “pull 1 stop” could be requested. Or, if exposed at ISO1600, push 2 stops. As with everything in photography, there is always a trade off with any changes you make, and no rules are cast in concrete! You don’t necessarily need to push or pull when the film has been exposed at different speeds – I’ll look at this now in more detail in relation to different film types.
Colour negative film for process C41
Colour negative film is so flexible that many commercial C41 processors don’t even have the facility to change the processing time (but our processors at Ag do have this facility). This is because the impact of adjusting the development time actually isn’t all that noticeable – you can “get away”, quite impressively, with re-rating colour negative film without adjusting development to compensate. A good example of this flexibility is where many photographers will routinely expose film a bit slower so as to deliver better shadow detail. For example, shooting, say, Kodak Portra 400 at 200 or 250 and giving normal development, would improve shadow detail. There would be no real adverse impact on the highlights and it would be very easy to control the bright areas, especially if the negative is being scanned. Even exposing this film at ISO100 would be easy to control. Conversely, we have customers who have recovered usable images after underexposing Kodak Portra 400 at ISO1600. However, with negative film, it is far better to err on the side of overexposure than under. This is because if you don’t get sufficient exposure of the shadow areas, no amount of additional development will make that detail appear! but you will increase the negative contrast. So our standard guidance on this is, only under expose, up-rate the ISO, if you simply don’t have enough available light.
We think it’s a bad idea to under expose and up rate the film speed for apparent creative reasons, even though there are plenty of opinions on the internet suggesting this is a good idea. The reason we disagree is because if you have a really well exposed negative in the first place, you can impose whatever creative style you like on it after you have scanned it – If you up-rate the film, you will have poorer quality negatives as a consequence, in terms of detail (regardless of whether it is push processed to compensate) and an underexposed negative is just harder to work with. It’s easier to impose your creative style on a scan file from a well exposed negative.
Traditional Black & White film
For black and white film increasing or decreasing development time has quite a significant impact on negative contrast. Consequently, pushing and pulling may also be useful for B&W film even if the exposure has been at normal ISO. For example, think of an image taken on a very dull and overcast day. The contrast of the scene might typically be really quite low in these circumstances, therefore, giving the film a bit more development will increase contrast and make the resulting negative easier to work with, particularly if it’s being printed traditionally, but also if it’s being scanned. Conversely, an image shot in full sun would likely be very high contrast and pulling development will help reduce contrast and, again, make the negative easier to work with.
Obviously, from a practical point of view, a typical roll of 35mm or 120 film might have a wide variety of images shot in differing lighting conditions, so, it usually pays to simply choose normal development. However, if you know the majority of images are, say, low contrast, a push could be a good idea. But, as with colour negative film we strongly advise against pushing or pulling for the sake of it. Or because someone on the internet says it’s a good idea! With black and white, especially, the best thing you can do is experiment with one film and one developer (or one processing lab if you are having your processing done by a lab – make sure they have a very consistent process, like we do at Ag!) and get to know that combination thoroughly – shoot many rolls over a period of time. That way, when you then decide to diverge and try a different film, you’ll be able to see the effects clearly. And when you do diverge, change one element at a time: try a different film. Try rating the same film at a different speed. etc.
As with Colour film, if you simply don’t have enough available light you may be forced to up-rate your film speed, and therefore you should order a push process in these circumstances. As a general rule, faster films push better – Ilford HP5 and Kodak Tri-X (both ISO400 films) are very popular films to up-rate as they respond well. But we would recommend pushing a maximum of 2 stops, to 1600, 3 stops to 3200 if you want to take a bit more risk. You can expect very acceptable results when pushing 1 stop to 800.
Colour Reversal film, process E6
Colour reversal, or slide, film is extremely sensitive to incorrect exposure. If you don’t pay close attention when taking meter readings or to your camera’s exposure meter – and interpret the readings intelligently – you stand the risk of making expensive mistakes: slide film and processing is by far the most costly form or analogue photography. Get it right, and you’ll be rewarded with stunning quality images that are impossible to match with any other medium. Get it wrong, and your images will be very disappointing.
It follows from this, therefore, if you have exposed the film at a different speed, it is absolutely essential you specify this to us when we process for you. Exposing, say, an ISO100 film at 200, and then giving normal development will yield excessively dark and probably unusable images. If you went the other way, and exposed the film at ISO50 and failed to pull development, you would have slides with excessively blown out highlights that are very hard or impossible to repair digitally. However, whilst slide film is very unforgiving, it does respond superbly to changes in exposure ISO coupled with requisite changes in development. An ISO 100 film accurately exposed at 200 and then push processed 1 stop will yield excellent quality results.
General notes for push/pull at Ag Photolab:
- We do not have the option to push/pull in less than 1 stop increments. We don’t support push or pull 0.5 stop, for example. We get asked for this from time to time (even “push 0.25 of a stop” sometimes) and it’s not worth the trouble, outside of a scientific experiment! If you shot your ISO400 film at 600, don’t worry, or simply order push 1 stop to be on the safe side.
- If you’re new to film, you need to remember that development given to a roll of film will be the same for all the images on that roll – you can’t process some frames at normal, some push 1, some push 2 etc.! May sound obvious to more experienced users, but if someone has only ever used digital photography, where you can select a different ISO from image to image, they could be forgiven for thinking this. Analogue is much more disciplined – and we think this is a good thing!
- Maximum pull for E6: 1 stop
- Maximum push for E6: 2 stops
- We do not pull process C41, there is no point in varying development with colour negative film when it’s been over exposed by 1, 2 stops. If you over exposed by 3 or more stops you’ll get away with it, maybe quite well. But there can be colour shifts.
- We can push C41 maximum 3 stops. But, note above, as to the impact it has being not hugely discernible
- Maximum pull for B&W: 2 stops
- Maximum push for B&W: 3 stops.
- If you want to push or pull B&W beyond 2 or 3 stops, there are specialist B&W developing formulas around, but this is outside the scope of a commercial processing lab as it would be too costly for us to offer. Definitely something for the home darkroom.
How to order Push or Pull processing at Ag
When selecting your processing options under C41, E6 or Black and White categories use the drop down box to select push or pull and number of stops. Also, mark your film “Push” or Pull” etc. so we know. If you are sending more than one film for processing and not all require a change in development, select the options differently for the other films and add to cart separately. We do levy a small additional charge for changes in film development as it is a little more work for us.