Jim Best-Devereux

Introduction

With the advent and growth of digital sensors film photography, once a titan of imaging within the United States and the world, began to become more and more obscure as the years pressed on. But to this day the allure of film captures the interest of amateur and professional photographers alike. Just as digital sensors are characterized and described, so too did film include much of the same analysis of its characteristic curves, MTFs, and spectral-sensitivity. Each type of film acted like a completely different sensor, generating different perceptions of the same scene. This “feel” that film generated is what draws so many to it still. Recreating this perception has led to many software tools ranging from image filters on the likes of Instagram and VSCO, and to attempts to faithfully recreate the properties of film in a digital world like DxO Filmpack. While these filters are great tools, they do not impact the image at the time of capture and do not quite truly recreate what so many seek when shooting with film.

In addition to software tools some digital sensors have been created that miimic the dynamics and principles with which film began with in the past. Sensors like Foveon’s X3, a stacked sensor that used the penetration depth of different wavelengths of light at a pixel to determine color, operates much the same way an image would be captured on film. Just as the Foveon sensor depends on the penetration depth of photons on the digital sensor so too did film depend on the same penetration of light passing through layers within the film and exciting the light sensitive silver crystals embedded in the layers. Through the use of a simulated Foveon sensor this work aimed to recreate the filter spectra that would produce images as if they were captured on a specific type of film.

Background

When aiming to simulate the characteristics of film it is important to understand how a color film image is created. Seen below is an image highlighting the structure of the color film and the layered structure it has.

Within the film there are embedded silver crystals that are light sensitive. When they are exposed to light they react, and later when exposed to developing chemicals in the film processing pipeline the crystals are washed away out of the image. For a black-and-white image this is how the negative is captured, and the exact characteristics of the image and its reactivity is based around the chemistry of the substrate and the crystals themselves. For a color image, these silver crystals are paired with a dye particle. When the silver crystals are washed away so too is the pigment, resulting in the color negative.

The colors formed in the color negative film are based on the subtractive color formation system. The subtractive system uses one color (cyan, magenta or yellow) to control each primary color. Unlike a black-and-white negative, a color negative contains no silver and the negative is a color opposite of the scene captured. They are a color negative in the sense that the more red exposure, the more cyan dye is formed.

Each kind of film has a specific set of characteristics much like the digital sensors covered in this class. Two examples of the curves for different types of film are as below. Kodak Portra 400 and Kodachrome 200 are very popular types of film that are still used extensively today.

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