Pushing and pulling films

By Nelson Tan

This article is for suitable for the more advanced readers. It covers a hotchpotch of information about films, such as film latitude, highlight and shadow details, contrast, reciprocal law failure, clip tests and dummy rolls.

(Any reference to films being push-processed pertain to slide films, not negative films.)

Reciprocal law failure

In real life, reciprocal law failure means that you like a girl but she fails to reciprocate your feelings (where's your sense of humour folks?). But in photography, reciprocal law failure means that film do not react in the same manner for extremely long or extremely short exposures. For example, when shooting night scenes, the meter gives a reading of 2 seconds at f/11. Using conventional calculations of doubling the exposure when you stop down by one more stop, you should expose for 4 seconds at f/16 right ? But because film become insensitive at long exposures, you need to exposed the film for a longer duration (eg. 2 seconds more) to compensate for this reciprocal law failure.

Reciprocal law failure when pushing films :

The same thing happens when you push a film beyond two stops. Many film do not react according to the reciprocal law. That is, if you exposed a ISO 100 film at EI 800, pushing it by 3 stops may not lead to a properly exposed image. You may need to process it at EI 1000 to get a properly exposed film. This is due to reciprocal law failure, and you may need additional processing time to compensate for it. Different films react differently, so you'll have to test individual films for their different reactions. Beyond 1-2 stops of pushing however, some films take on slight colour casts (eg. magenta). So try not to push your film (and your luck) too much. What if you can't afford to test because the roll contains important images, and you can't risk processing it at the wrong speed ?

Clip Tests/Dummy Roll

This is where clip tests and dummy rolls come in. Clip tests are what the name suggests - the lab snips off around 2-4 frames from the roll (called a clip) and process it at your indicated push/pull (hence the "test"). Using the results of the clip test, you can either increase or decrease the processing time. You lose the first few images, but you can accurately determine the processing time of the rest of the rolls to ensure usable images.

Dummy roll is where you shoot another roll of the same type of film at the same conditions using the same settings. This will create another roll that has identical exposure as your important roll. Process this roll at your indicated push/pull timing, and use the results to determine the processing time for your important roll. This method is slightly less accurate than the clip test, since it's difficult to replicate the original exposure, but you will not lose any of the frames of the original film.

Film latitude, highlight and shadow details, contrast

Film latitude refers to the range of highlights and shadow details that the film can hold. A film with wider latitude can hold a wider range of highlight and shadow details than a film with narrower latitude.

Highlight and shadow details are details in the highlights and shadow respectively. For example, if you can still see the texture of white snow in the image, there is highlight details. If you can still discern the plants in the shade of the trees, there is shadow detail.

In this example, this picture was taken in Yosemite National Park under overcast skies, which lowered the contrast of the scene. By slightly underexposing the image, I have enriched the colours of the leaves of the ground, and prevented the reflections on wet granite rock (foregound) from totally burning out.

(See section on exposure)

Thus, it goes to say that a film with wider latitude is lower in contrast, and the vice-versa is that a narrow latitude film is higher in contrast. Films like Velvia have high contrast, and lose highlight and shadow details easily. Highlights become burnt out into featureless blobs, and shadows become a patch of dense blacks. Low contrast films are wide in latitude, and you can retain highlights and shadow details much better. An example of low contrast film will be Fuji NPH ( a print film).

Print film versus Slide Films

Print films can record a range of around 7-8 stops (from highlight to shadows), while slide films can only record a range of 4-5 stops. To test for yourself, choose a contrasty scenario where there is a wide range of bright highlights and dark shadows. Shoot the scene using both print and slide film, and examine the slide and the negative (not the print) for highlight and shadow details. You will find that the negative has a much wider range of details than the slide film. Thus, when faced with a contrasty scenario, a print film will be preferable to slide film.

Exposure

This is something technical, and has to do with darkroom dynamics:

When shooting print films, make sure that you expose for the shadows. It is possible to burn in overexposed highlights from the negatives, but clogged up shadows contain no details, and no darkroom magic can salvage a good print from it. So if you exposed for the shadows, you are ensured that you can print the details in the shadows, and burn in the burned out highlights in the darkroom.

The reverse is true for slide films. You must expose for the highlight details. This means that the highlight must not be so bright that it is more than 4 stops from your exposure. You can print in the shadow details, but a featureless highlight is a forgone cause. There is nothing you can do to squeeze out any details from a burned out highlight.

Knowing that, you should:

- expose for the shadows when using print films

- expose for the highlights when using slide films

The logical thing to do is:

- Never underexpose print films

- Never overexpose slide films

If you underexpose print films, you will not get the necessary shadow details. But given the wide latitude of print films, you can overexpose print films by 2 stops and yet get great results.

If you overexpose slide films, you will not get the necessary highlight details. But by underexposing slide films by 1/3 to ½ stop, you may in fact get slightly richer and more saturated colours.

Altered processing and contrast

Altered processing, i.e pushing and pulling can change the contrast of film. When a film is pushed processed, the contrast is increased. The vice-versa is true of lowered contrast when the film is pull-processed. Professionals sometimes deliberately push or pull the films by ½ stops to a full-stop to gain extra contrast or to lower the contrast of films. Pushing burns out the highlight, while pulling the film brings out the shadow details. Thus, altered processing is not just for times when you set the wrong film speed. It can be used for creative purposes. Also, since pushing increases graininess of the film, pros use pushing to achieve a more grainy image.