Flatness of film negative macro reproduction stand: oil-free pressing glass vs Newton ring-free glass (ANR Glass)

1. The physical fate of film curling and the zero tolerance of macro depth of field

Why is it difficult for film to remain flat naturally after being developed and dried?

• The shrinkage rate of the emulsion layer and the film base is asymmetrical:
• The negative film is made of cellulose triacetate or polyester (PET) plastic base and a natural gelatin film on the surface;
• During the drying process, the gelatin layer loses water and shrinks violently, while the size of the plastic base remains almost unchanged, resulting in huge asymmetric tension, forcing the film to naturally curl toward the emulsion surface into an arched tube shape (Cupping/Curling);
• Millimeter penalty for macro lens depth of field:
• When reproducing full-frame negatives at 1:1, even if the lens is contracted to the best sharpness aperture of f/8, the optical depth of field is only less than 1 mm (approximately 0.8mm)!
• If the center of the film is arched by 0.5 mm, after you focus sharply on the center of the film, the four corners of the film will have completely drifted out of the focus plane, causing the edges of the scanned digital file to collapse and blur. External force must be used to force the film flat!

2. The causes of optical interference physics of Newton's Rings

Many creators find two pieces of clean ordinary frameless picture frame glass and clamp the negative flat. As a result, after scanning, a dazzling colorful ripple circle appears in the center of the picture:

• Thin-Film Optical Interference:
• The surface of ordinary float glass is highly ground and polished, and the surface is absolutely smooth macroscopically;
• The unexposed glossy base of the negative is also extremely smooth;
• When the two are closely attached together, due to the tiny undulations at the microscopic molecular scale, there is an extremely thin wedge-shaped gap of air with a thickness ranging from hundreds of nanometers to several microns (exactly equal to the wavelength of visible light) sandwiched between the two highly smooth surfaces**;
• Destructive & Constructive Interference:
• When the scanning light source penetrates the glass, two reflections occur on the lower surface of the glass and the upper surface of the film;
• The optical path difference between these two microscopic reflected light waves is exactly equal to odd or even multiples of half wavelength, causing violent physical fluctuation interference;
• The air films of different thicknesses in the space disassemble the white light into circles of concentric circles, like high contrast color interference fringes (Newton rings) like rainbow water ripples! Once generated, post-processing software cannot separate and remove it from the real image below.

3. The salvation of nano micro-carvings of ANR - Anti-Newton Ring Glass

The ultimate industrial solution to interference fringes is to use professional ANR glass:

• Single-sided micro-acid-etched and micro-frosted physical structure:
• One side of the ANR glass has been lightly etched and ground with high-precision chemical hydrofluoric acid;
• Observed under an electron microscope, its surface is distributed with millions of extremely tiny, uniform, micron-level Micro-Etched Texture;
• Elimination of continuous uniform air film:
• When the side with microscopic particles is lightly pressed against the smooth base surface of the film, tens of thousands of microscopic raised points evenly support the film;
• The two are forcibly separated, and the thickness of the air gap is randomly disrupted and broken. It is absolutely impossible to form a continuous and uniform wedge-shaped air film that produces optical path coherence;
• The thin film interference conditions are completely physically destroyed, and Newton's rings are 100% completely dissipated instantly!
• Transparency Preservation: The acid-etched particles are extremely small, much smaller than the size of the pixel sensor of a digital camera, and will not leave any matte impurities, noise, or sharpness attenuation on the scanned film.

4. The golden rules for assembling the high-contrast oil-free remake stand and placing the front and back sides

Set up a commercial-grade digital camera film reproduction system (such as using Valoi / Negative Supply / homemade reproduction stand):

1. ANR glass front and back judgment (life and death touch):

• Gently touch both sides of the glass with your fingertips: one side is extremely smooth and cool, and the other side has an extremely subtle, silk-like damping feel**;
• Iron Rule of Installation Orientation: The side with the frosted feel must face the negative film and be close to the smooth base surface of the negative film! If placed backwards, it will have no anti-Newton ring effect.

2. Guidelines for positive and negative film orientation:

• The emulsion layer (matte rough surface) of the film itself has a microscopic gelatin texture and naturally does not produce Newton rings;
• Newton rings will only occur on the smooth back side of the film base;
• Therefore, the system only needs to cover one side of the smooth film base with a piece of ANR glass; or use an upper and lower double-layer clamp (ANR glass on the top, high-transparency glass slide on the bottom), and use a high display index (CRI > 97) LED flat-panel backlight to achieve 100% absolute flatness of the entire film and full sharpness in the center of the edge. Ultra-clear scanning of billions of pixels!

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