Digital Film

PSA Journal, August, 2001 by Albert J. Klee

"O Villain! Thou hast stolen both mine office and my name"

Although I apologize to the Bard for filching the above quotation from The Comedy of Errors, a long-time annoyance to me has been the term, "digital film," used for the removable storage cards for digital cameras, such as CompactFlash and SmartMedia. For one thing, traditional film both records the image and stores it; "digital film" only stores the image. Film photographers have learned to deal with the nature of film and understand the jargon of terms such as grain, contrast, resolution, latitude, sharpness, saturation, etc. Digital is the new boy on the block, however, so I propose in this article to highlight some of the major differences between the two.

* The Basics

Traditional and digital cameras are based on similar principles: gathering light, recording light, and transforming that light into an image -- that is the very nature of photography. The basic distinction between the two is that film cameras rely on a chemical reaction between light and film, while digital cameras rely on an electrical reaction between light and a photosensitive diode.

As most photographers know, black and white film employs silver-halide crystals to record the intensity or brightness of light. When the film is exposed, the light changes the crystals chemically, causing them to bind together in clumps. The more light that hits the film, the greater the clumping. The film now contains a chemical record of the light, which then is translated into an image during the developing and printing processes.

Digital cameras, on the other hand, use a solid-state device called an image sensor. These fingernail-sized objects contain millions of photosensitive diodes called "photosites." Each photosite is one element of the whole picture that is formed, thus it is commonly called a picture element, or "pixel" for short. When the shutter is open, each photosite records the intensity of the light that falls on it by accumulating a charge; the more light, the higher the charge. The charge recorded by each photosite is then stored as a set of numbers that can then be used to set the brightness and color of dots on a computer screen or printed page to reconstruct the image.

* Resolution

The resolving power of film is measured simply by its ability to reproduce patterns of horizontal and vertical black lines on a white background as lines and not as uniform gray areas.

In a digital image, resolution refers to the number of pixels in the image. When a relatively small sensor array is used to create an image, the individual pixels become very apparent in the resultant picture. A common example is the jagged stairway effect produced on diagonal lines. (Figure 1). In film, sharpness, although allied to resolution, is not the same thing since it relates to the transition between a light tone and an adjoining darker tone (i.e., acutance), and this does have a direct equivalent in digital images; "resolution," however, does not.

[ILLUSTRATION OMITTED]

* Color - Film Versus Digital

The first photographs could only record black and white images, but a major breakthrough occurred with James Clerk Maxwell's discovery in 1860 that color photographs could be formed using red, blue, and green filters. A tartan ribbon was photographed three times, each time with a different one of the color filters over the lens. The three images were developed and then projected onto a screen with three different projectors, each equipped with the same color filter used to take its image. When brought into register, the three images formed a full color image.

Today's color film works in much the same way. There are three emulsion layers, each one responding to a primary color. The bottom layer reacts to red, the middle layer to green, and the top layer to blue light. Coupler dyes, each one designed to react with only one of the emulsion layers, combine with the silver-halide crystals. The resulting red, green and blue, mix to approximate the actual color of the light that first hit the film.

Although filters are also used with digital sensors, the methods vary, including the following:

(a) Three separate image sensors can be used, each with its own filter. This way, each image sensor captures the image in a single color.

(b) Three separate exposures can be made, changing the filter for each one. This way, the three colors are `painted' onto the sensor, one at a time.

(c) Filters can be placed over individual photosites so each can capture only one color.

However, method (a) is expensive and method (b) slows down the process. Therefore, most consumer-level digital cameras use method (c).

However, when small filters are placed directly over individual photosites on the sensor, the optical resolution of the sensor is reduced by one-third. This is because each of the available photosites records only one of the three colors. On the typical sensor there are twice as many green photosites as red and blue. This is because human vision, which we want to emulate, is most sensitive to the green wavelengths, and also because the green readings are used to compute brightness.