Question: What is the difference between silver halide and digital image capture?
Answer: In both cases, you have a device or material that is capable of sensing photons of light that are reflecting off people and objects in front of the lens. With film, photons strike silver halide crystals suspended in an emulsion. The energy transmitted by the photons alters the structure of the crystals, forming a latent image. Chemical development transforms the latent image into a negative image on the film. When photons strike an electronic sensor, such as a CCD (charge coupled device), electrons are also generated. But instead of "recording" a copy of the actual image like a painting on a canvas, some sort of counting device records electrical currents that represent images.

Question: Does this mean that CCDs essentially mimic the functions of silver halide crystals?
Answer: There are critically important differences. For example, every frame of film contains billions of silver halide crystals. Each one of those crystals is uniquely shaped, and they are scattered randomly in the emulsion. That, in part, accounts for the unique texture of film, which we associate with fantasy. CCDs, on the other hand, use a fixed structure to collect information. The sensors are arranged in a matrix. The result is a distinctive look that we tend to associate with live newsgathering.

Film performs two functions: it both captures and records the images. With CCDs, you need a separate recording device, such as videotape to record images. This is a primary factor affecting the quality of digital image capture. For video to record the same dynamic range as film, you would need a record head with an extremely tiny gap, and the tape would have to move at extraordinary speeds. Those are formidable technological hurdles that nobody has figured out how to overcome in a practical manner.

Question: How does the result affect the image capture ability of video devices?
Answer: It is physically impossible for them to capture all the image that we expect to see on film. The shortfall shows up primarily in color resolution. The eye is most sensitive to green. So video-capture devices under-sample, by about half, in the red and blue pan of the spectrum, where the eye is more forgiving. In the captured image, this shows up as a lack of texture. Look at the flesh tones in a video image, and you'll see that it does not capture the subtleties of variations in tones that we expect to see across the entire image. I'm not a cinematographer, so I can't address this as an artistic issue, but we know that these subtleties in color tones, contrast and textures are important components of the visual grammar of flimmaking. They are part of a silent language, which evokes emotional responses. You can both see and feel the difference in emotional content.

Are there other distinctions between film and electronic images?...
What if the film is being produced for television?...
Some people claim that HDTV is film-like. Would you agree?...
Will HDTV inevitably close the gap in image quality?...

Thomas G Wallis is Worldwide Technical Director and Vice President of Entertainment Imaging.

Reprint with permission from InCamera magazine April 1999