Practical Contrast Control of Film Push and Pull Processing and Formula for Calculating Extended Development Time

1. The underlying physical nature of collision and desensitization: the dark parts are determined during shooting, and the highlights are shaped in the darkroom.

The iron law of physics at the core of the film industry: "Exposure determines shadows, and development determines highlights":

1. Push Processing is not a magical sensitivity jump:

• Expose ISO 400 film at ISO 1600 (underexpose 2 stops):
• At the moment when the shutter is pressed, because the amount of light entering is reduced by 75%, the silver halide crystal in the extremely dark part does not receive enough photons at all, and no photosensitive latent image center is formed at all! **
• No matter how long you soak in the developer, the details in the dark areas cannot be changed out of thin air, and the dark areas will be completely black;
• Extending the development time is essentially to make the silver crystal clusters in the highlight and middle gray areas that have been weakly photosensitive develop crazily, and forcibly raise the middle gray to a brightness visible to the human eye!
• By-products: Contrast increases sharply, grains become thicker, and black and white edges are extremely impactful.

2. Physical mechanism of pull processing:

• Expose the ISO 400 film at ISO 100 (overexposure by 2 stops) and shorten the development time by 30%;
• The shadows are full and delicate, and the highlights are stopped by stopping the developer brake before they have time to become white and agglomerate. The picture is extremely smooth and soft.

2. Classic mathematical model for extending the development time of black and white film

If the official standard development time is ( T_0 ) (20℃):

• Push 1 Stop / 400 to 800):
  👉 Development time extended to ( 1.35 imes T_0 ) (35% increase in time);
• Push 2 Stops / 400 to 1600):
  👉 The development time is extended to ( 1.80 imes T_0 ) (80% increase in time);
• Push 3 Stops / 400 to 3200 limit):
  👉 The development time is extended to ( 2.30 imes T_0 ) (increased by 130% time). It is recommended to switch to a special push potion containing phenidone (such as Microphen).
• Desensitization -1 level (Pull 1 Stop / such as 400 to 200):
  👉 Development time reduced to ( 0.75 imes T_0 ) (25% time reduction).

3. Decision-making on shooting scene theme matching

• Continuously cloudy sky, flat light without shadow, gray and lifeless picture:
  👉 Decisive Charge +1 or +2: Artificially increase the contrast, making the cloudy day dramatic and three-dimensional!
• Scorching sun at noon in summer, large light ratio scene in snow-capped mountains and deserts:
  👉 Decisively overexpose by 1 stop and desensitize (Pull 1) flush: Compress the dynamic range and save the details of bright clouds.

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