Push and Pull Processing Films to change the ISO

Push and Pull Processing Film


Higher effective ISOs allow photographers to expose images at reasonable shutter speeds and moderate f-stops. Push-processing is one technique. The practical limit is one or two f-stops to minimize loss of quality. You can push more if you are willing to accept greater losses. On the minus side, there is bound to be some grain build-up. Whether it is objectionable or not is a matter of judgment. Also, increases in contrast in shadow areas leads to significant detail loss.


Increasing effective ISOs calls for processing compensation. With slide film, first developer times must be increased; for black-and-white film, the basic developing times need to be increased. Table below gives some guidelines for general-purpose developers such as D-76, used in conjunction with general-purpose black-and-white films.


Exposure Modifications When Using General-Purpose Developers

 

Change in Rated ISO      

Exposure Change Desired

Change in Development Time

Black-and-White Film

 

 

2x          

-1 f-stop

+25%

4x

-2 f-stops            

+75%

8x

-3 f-stops            

+150%

1/2x      

+1 f-stop             

-30%

1/2x      

+2 f-stops           

-30%
And Dilute by 50%

Color Negative Film

 

 

2x          

-1 f-stop              

+30%

1/2x      

+1 f-stop             

-30%

Color Transparency

 

 

2x          

-1 f-stop              

+25 %

4x          

-2 f-stops            

+75 %

1/2x

+1 f-stop

-25%

1/2

+2 f-stops           

-50%

 

Pulling Black-and-White Film

  • Reducing effective ISO has two effects. First, excessive contrast can be reduced, especially in the vital shadow areas. This is done by lowering the shooting ISO and decreasing the developing time by specific amounts. A further effect of this technique is to reduce apparent grain well below what is expected. For decades black-and-white photojournalists rated Tri-X between ISO 250 and 320 instead of its standard ISO 400. When the de-rated film was given about 20 percent less exposure, visible grain decreased. One should be careful not to overuse pull-processing in situations where the subject exhibits normal or low contrast, for flat, dull negatives usually result.

Pushing and Pulling Color Negative Film

  • For the most part, either pushing or pulling most color negative films produces distinctly inferior negatives. Push-processing print film results in unsatisfactory color reproduction with an imbalance of colors in different portions of the negative. Trying to bring the colors back into balance using color printing filters in the darkroom is almost always a frustrating experience. As soon as you balance one portion of a pushed negative, another goes out of balance. Additionally, image contrast is adversely affected.

  • Pulling is even less desirable. The worst effect is a drastic reduction in negative contrast. If you use general-purpose color negative film, you don't really need either push or pull. Negative films possess impressive exposure latitude, sometimes exceeding 6 f-stops. Taking this into account, it is plain to see that it is questionable to employ either technique.

Color Transparency Films

  • All color slide films, including Kodak-chrome, are amenable to some extent to pull- and push-processing. The push/pull range is from about +2 f-stops to -1 f-stop. Increasing effective ISO by increasing first developer times almost always results in grain and contrast buildup. If the push is moderate, grain and contrast can be kept under control. Color will begin to shift usually toward red almost immediately even with 1/2 f-stop push-processing.

  • Some films, such as Fujichrome 1.00 and Agfachrome RS 100+, are particularly well suited for pushing. You will incur a minimum performance penalty if pushing is limited to no more than one f-stop. It is only when you choose to go beyond one f-stop that things get a bit dicey. However, if you need extra speed and pushing the film is the only option open to you, then the compromise in film performance may be acceptable. When you pull slide film, the result is an overall lowering of image contrast, distorted color, and marginally finer grain. Expect an overall color shift toward blue. Pulling slide film might be the only viable technique to tame extremely high contrast lighting where important visual information is contained in both shadows and highlights. Pulling is only practical if high color saturation is not a primary goal. Pull-processing invariably leads to muted, distorted colors.

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