This post sets out my understanding of push/pull largely as a way of making sense of it for myself.

Sources of confusion

One source of confusion stems from the fact that the words ‘push’ and ‘pull’ are used inconsistently.

Many (most?) film photographers do not print their pictures, choosing rather to scan their negatives. The film scanner’s ‘push and pull’ stops at the film development stage. In the darkroom ‘push and pull’ stops at the final print stage. The darkroom worker has to develop the negative to a state that fits into a paper’s tonal range capability, which is narrower than film. An ‘optimal’ negative destined for a chemical print is not the same as an ‘optimal’ negative destined for a scan.

The situation is further complicated by how digital photographers use these words ‘pull’ and ‘push’ to mean ‘over-expose’ and ‘underexpose’, and simply turn the ISO button to achieve this, either to gain a faster shutter speed or greater depth of field or as an aesthetic choice.

A second source of confusion comes from a misunderstanding that many 35mm film photographers seem to have. On a recent photo-walk with a film photographer, I asked what speed he was rating his film, TMax 400. ‘Oh, box speed’, came the reply, ‘I don’t push and pull develop as that isn’t an option when using roll film’. (Just to explain – a roll film can only have one development regime; it isn’t practical to develop each frame according to that frame’s need).

But finding a personal film speed and push-pull development are two completely different things. The distinction is important.

A film comes with the manufacturer’s recommendation of its speed, called ‘box speed’. So, Ilford HP5 Plus is rated at a speed of 400 ISO by Ilford. Under laboratory conditions using specialist equipment, Ilford have assessed that a speed of 400 dialled into the light meter will give an exposure that will reproduce a middle-grey tone in a scene as a middle-grey tone on a dried print, say. But that will not necessarily be the case for us. Different equipment, conditions and process are likely to mean that the exposure required for us to achieve a middle-grey from that same scene would be different. This real-world exposure is called the effective speed (‘Exposure Index’ EI) for that film. Often the EI is less than box speed.

Finding a personal film speed has nothing to do with push and pull development. It is simply a calibration exercise that maximises your chances of matching the tonal values of a scene to the tonal values of a print. Pretend that manufacturers did not advertise box speeds. What exposure index would you dial into your meter? The answer is, the EI assessed from a personal film test. The fact that it is often impractical to develop each frame on a roll according to its need is irrelevant.

Pushing and Pulling

When learning the basics, I studied David Vestal’s book, The Craft of Photography’, one of my favourite ‘how-to’ books on B&W darkroom work.

On page 194 ¹ he says ‘In terms of shadow contrast I require in my pictures, the recommended exposure for Tri-X (ASA 400) is underexposure; therefore I rate Tri-X at EI 200 instead‘.

That is, Vestal overexposed his images to get the shadow detail he required, against the mighty advice of Kodak.

Pull/push film development is over-exposing/under-exposing film (compared to the ‘normal’ EI obtained in the personal film test) and applying a particular film development treatment. We normally under-develop and over-expose film, and over-develop and under-expose film, but that need not necessarily be the case (see later). Aesthetic choices apply. The reason why we normally under-develop and over-expose film, and over-develop and under-expose film is that this treatment best gives both shadow and highlight detail.

Shadow detail is mainly acquired through sufficient exposure of film to light. During the development of the film, once the available photon-induced halides have been reduced by the developing agent, little further development is possible as the number of photon-induced halides have already been converted. So, during development, shadow areas stop getting darker but highlight areas continue to develop as there is still an abundance of available photon-induced halides. To prevent the highlight areas getting too light (into paper white), development is curtailed with a stop bath.

It is this process that underpins the adage: ‘expose for the shadows, develop for the highlights’.

Pulling the development is to curtail it earlier than would be normal. You literally pull the negative from the development solution. So when we pull, we normally overexpose the negative to capture shadow detail and curtail development to prevent delicate highlights from being lost. We contract the tonal range relative to the subject brightness range of the scene.

Pushing development is the opposite. If the subject brightness range of a scene is narrow, that is the scene is flat, then we can under-expose the film and over-develop the negative. This expands the tonal range. However, this can come at a cost. Over-developing boosts the low-mid and mid tones and loses some shadow detail. It may also increase grain.

Sometimes pushing film is used as a practical step in taking a picture. Suppose you are in a dim bar taking pictures of a rock band. To freeze the action, you will need to increase the speed of your camera meter by rating the film at, say, EI 3200 (which will reduce the shutter time). This will help to freeze the action. To compensate, the film will need to be developed for a substantially longer time than ‘normal’. The final print is likely to be grainy and contrasty.

Pacticality and aesthetics often co-evolve. Street photography, war photo-journalism, concert photography are examples where the demands of the situation (speed of subject movement, dim light) have led to a ‘look’ of gritty realism.

Developers that increase speed

What does it mean to say that a particular developer increases speed? For example, Ilford’s Microphen gives an effective speed increase for HP5 Plus of at least half a stop whilst Perceptol leads to a loss in speed.

What this means is that we can rate HP5 Plus at an EI of half a stop more than our normal speed. So, for HP5, we can use an EI of at least 600 if our normal speed is 400, essentially without the losses that often accompany pushing film, such as grain. We use Perceptol at EI of 200 for HP5 but the gain we obtain is very fine grain and very smooth tonal gradation. You pays your money and takes your choice. There is always a compromise to deal with, normally one between acutance, shadow detail and the appearance of graininess.

Does the developer actually change the speed of the film? I assume it does by shifting the characterstic curve for that film to the left or right, rather than altering its slope. But this is a level of detail that I don’t need to get into here.

Strategies

I have talked about over-exposure/under-development (pull) and under/over (push) processing. These approaches are usefully seen on a diagram compared to other strategies:

The chart shows film exposure options against film development options. Pull processing is given by square ‘3’ and push processing by square ‘7’.

Option 3 compresses highlights so that a wide subject brightness range of a scene can be recorded within the limits of a negative/paper. This option is often used by landscape photographers trying to control contrast. Conversely, Option 7 expands the midtones and highlights and is often used by street photographers to enable them to freeze motion in dim conditions.

Mortenson thought that a derivative of option 7 (‘7D’) gave the best results in specified situations. Provided the light was right and coupled with a very long compensating paper development, 7D results in great highlight gradations.

Johnny Patience has argued that over-exposure coupled with over-development (Square 9) gives good results with modern films. I leave it to you to decide whether he is right.