Push / Pull Processing 

Eastman Kodak recommends a controlled ECN-2 process (as specified by the H-24 manual) for the optimum performance of all of its color negative films. However, it is recognized that creative control and/or physical limitations may dictate exposing film under less than ideal conditions (i.e. normal exposure). This may necessitate the use of push / pull chemical processing conditions to compensate for the non-standard exposure / speed conditions that are used for the film.

Normal ECN processing = 3 minutes (180 sec.).
This is the recommended processing time used for normal ECN exposure - (exposure of film at the rated speed index supplied by the film manufacturer)

Pull processing - compensation for over exposure of ECN film by underdevelopment of the chemical processing

  • Pull 1 = 2 minutes 30 seconds (150 sec).

push processing - compensation of under exposure of ECN film by over-development of the chemical processing
  • Push 1= 3 minutes 40 seconds (220 sec)
  • Push 2= 4 minutes 40 seconds (280 sec)

All processing conditions are to be operated @ 106F ( 41.1C). These Push / Pull conditions are to be regarded as starting positions for any laboratory. The times may be varied to achieve LAD densities equal to the original normally exposed negative.

Push Processing

The following comments provide additional sensitometric information relative to the film performance as a result of the application of push processing.

Push processing is achieved in the laboratory by reducing the transport speed of the processing machine, thereby increasing the development time. Push processing is often used to recover density from under exposed camera originals. Ideally, the density loss from under exposure is exactly compensated by the increased density provided by push processing. The compensation is not perfect however, and the final result is typically higher contrast images with a color bias and reduced photographic speed. Increased granularity is also a by-product of push processing. These effects are generally independent of film code or emulsion differences.

Sensitometry:

Contrast:
Contrast build up from pushed processing is generally most dramatic in the bottom (cyan and magenta) layers of negative film and least dramatic in the top (yellow) layer. This is because development is a diffusion limited process. Contrast mismatch problems are introduced with pushed processing resulting in high red to blue and high green to blue contrast. The relatively high red to blue and high green to blue contrast of the negative produces yellow highlights and blue shadows in the print when mid-scale neutrals are balanced. In many cases the yellow highlights may have a red bias because the mismatch tends to be higher in the cyan layer compared to the magenta layer. It should also be noted that contrast mismatch varies with exposure; stronger effects with over exposure, weaker effects with underexposure. Under exposure, combined with pushed processing, tends to lessen the effects of increased contrast from pushed processing alone. Differences in exposure and scene content may have a significant effect on the perception of color in the resultant print, when pushed processing is used.

Speed:
ER speed increases with Push processing may be significantly less than needed to offset speed loss from under exposure. With the given conditions above, Push 1 processing produces ER speed increases of less than a 1/3 of a stop and Push 2 produces ER speed increases of around 1/2 of a stop. Push 2 taken in combination with two stops of under exposure represents a stop and a half loss in real speed. The end result is smoky shadows in prints.

Granularity:
Regarding granularity, the following general comment is true: push processing increases the granularity for all films. Further, it is apparent that under exposure also produces increases in the granularity for all films. The combination of push processing and under exposure may produce additive increases in granularity up to the extent of 10-12 units, compared to normal exposure / normal processing. It seems probable that some customers might complain about grain (and color rendition) when comparing under exposed , push processed originals to normal exposure and normal processing conditions.

AMT:
AMT (sharpness) numbers decrease with under exposure with all films. The combination of under exposure and push processing produce lower AMT's compared to normal exposure, normal processing. The loss in sharpness from under exposure may not be objectionable in pictures however, because this loss may be compensated subjectively by the apparent increase in sharpness associated with the higher contrast from push processing.

Summary (Push Processing):
Push processing is not recommended as a means to increase photographic speed. Push processing produces contrast mismatches notably in the red and green sensitive layers ( red most) compared to the blue. This produces reddish- yellow highlights, and cyan- blue shadows. Push processing also produces significant increases in film granularity. Push processing combined with under exposure produces a net loss in photographic speed, higher contrast, smoky shadows, yellow highlights and grainy images, with possible slight losses in sharpness.

Pull Processing

Summary (Pull Processing):
In a similar analysis, pull processing combined with over exposure of ECN film, produces a noticeable improvement in granularity, a reduction in overall contrast, a slight color bias (yellow shadows and blue highlights) with no perceptible change in sharpness.

References 

  • Motion Picture Camera Techniques - David Samuelson
  • American Cinematographer - Rod Ryan
  • Motion Picture Film Processing - Dominic Case provide general guidelines for achieving pull and push processing conditions to compensate for over and under exposures on camera negative films. The following recommended conditions were derived from public domain information:
  • Eastman Color High Speed Negative Film 5293 by G. Kennel et.al., SMPTE Journal Oct. 1982
  • Graininess in Motion Pictures, 1984 SMPTE presentation by R. C. Sehlin for push and pull processing of ECN films.