Traditional black and white darkroom enlarger 'non-sensitized paper (paper white and D-max)' exposure step wedge test

1. The mathematical core of test strip testing: arithmetic exposure is wrong, geometric series exposure is the truth

When many darkroom beginners make test strips, they use a piece of opaque black cardboard to cover them and expose them in sequence: 2 seconds, 4 seconds, 6 seconds, 8 seconds, 10 seconds (arithmetic sequence):

• Logarithmic Weber-Fechner law of human visual perception:
• The human eye’s perception of changes in light and shade increases geometrically;
• The exposure increased from 2 seconds to 4 seconds (doubled, increased by a whole stop of exposure), and the contrast of the picture changed drastically;
• But going from 8 seconds to 10 seconds (an increase of only 25%, less than a third of a stop), the difference between light and dark is negligible! The arithmetic exposure completely loses the test resolution at the longer end.
• Scientific Geometric Stepping Method:
• Set the basic reference time to 2 seconds;
• The occlusion step time should be as follows: 2 seconds, 2.8 seconds, 4 seconds, 5.6 seconds, 8 seconds, 11 seconds, 16 seconds (incremented by (\sqrt{2}) or half-step ratio);
• The light and dark contrast steps between each test strip are spread out strictly and evenly, and the optimal exposure window can be locked at a glance.

2. Extreme black saturation point (D-max): a physical measurement that produces the minimum blackness

An excellent black and white photo must have calm and deep pure black (Zone 0), but overexposure will completely obscure the dark details:

• D-max physical saturation limit of photographic paper photosensitive silver salt:
• The total amount of metallic silver that can be reduced in the emulsion layer of photographic paper is limited;
• When the exposure time reaches a certain critical value, all the silver bromide crystals on the surface are reduced to dense metallic silver, and the photo paper presents Maximum Density/D-max with the lowest physical reflectivity;
• If you increase the exposure time thereafter, the black will never become darker, it will only compress the middle gray level needlessly.
• Measure the film base exposure time (MTMB practical operation):

1. Load the blank edge of the transparent film (Film Base + Fog) that has been completely developed but not exposed to light into the enlarger film holder;
2. Open the enlarger aperture to f/8;
3. Lay a narrow strip of photo paper flat on the base plate, and use black cardboard to make step exposures at intervals of 2 seconds (2s, 4s, 6s, 8s, 10s, 12s, 14s);
4. Complete development for 2 minutes, fix, and blow dry (it is strictly forbidden to look at wet photo paper! Photo paper will produce a 'dry-down' effect when it is wet, and the blackness will be 10% darker after drying than when wet);
5. Observe under bright sunlight: Find the smallest time frame (for example, 8 seconds) that just reaches the deepest pure black, and the blackness will not change at any later time! This time is the standard exposure baseline for this set of medicines and this film system.

3. Contrast filter (Multigrade Filter) and semicolon test

After determining the basic exposure time, how to determine whether the picture contrast is hard (No. 001) or soft (No. 45):

• Observe the precipitation of high-gloss white (paper white Zone X):
• Print the photo at the base time and observe the areas in the picture that should show very light texture (such as white walls in the sun, white bride's gauze);
• If the highlights are as white as chalk and have no details: It means the contrast is too hard, you must use a low-contrast filter (lower the semicolon, such as cutting to 1.5), lower the contrast and let the highlights shrink;
• If the highlights are dark, gray and turbid, and the entire film is gray and weak: It means the contrast is too soft, and you must use a high-contrast filter (raise the semicolon, such as No. 3.5 or No. 4) to increase the contrast between light and dark!

4. 10% pre-judgment compensation for Dry-Down Effect

The dividing line between darkroom masters and amateur enthusiasts lies in the fine-tuned control of the physical shrinkage of photographic paper during drying:

• Wet and Dry Drift in Reflectance and Gelatin Gloss:
• The surface of wet photo paper is covered with a smooth water film. The high refractive index of the water film makes it easy for light to penetrate, and the dark parts look crystal clear and have excellent contrast;
• When the photo paper is dried, the water film evaporates, and the microscopic surface of the gelatin produces tiny rough undulations. The weak diffuse reflection of the diffused light makes the highlights dim and the shadow contrast slightly rebound and become lighter by about 8% to 12%!
• Practical formula for prejudgment compensation:
• When you look at the perfect photo in the fixing tank in the darkroom, it will often look slightly darker and dull after drying;
• Dry Shrinkage Compensation Rule: After determining the optimal exposure time for wet photo paper, actively multiply the enlargement time by 0.9 (reduce 10% exposure time) during the final full exposure!
• The finished photos after drying are displayed under spotlights in the gallery. The highlights are pure and transparent, and the shadows are deep and tangible, achieving a master-level perfect balance.

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