film vs digital imaging from KODAK'S web site

[Greg Crawford]

A Technical Comparison, Film and Digital Imaging


What are the differences between film and digital imaging technologies? How do those differences affect the way audiences perceive images? Cinematographers and directors use words like "dream-like" and "organic" to describe the film look. Marshall McLuhan, a Canadian sociologist, took a scientific approach to understanding the fundamental differences in his 1964 book Understanding Media. McLuhan concluded that we process film and video information differently. He labeled video images "cold" and film images "hot."

What happens when light strikes a frame of motion picture film? How are images formed? How does that differ from the way digital video cameras record pictures? Which format has higher resolution, 24P HD or Super 16 mm film? We asked Dick Sehlin, chief technology officer and vice president of Kodak's Entertainment Imaging division, these and other related questions. Following are excerpts of the answers:
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QUESTION: What are the differences between the ways film and digital video record images?

SEHLIN: Every frame of color negative film consists of multiple layers of emulsions that are individually sensitive to red, green or blue light. Each layer contains literally billions of patented T-Grain silver halide crystals that are embedded in a gelatin emulsion in a random pattern. The tabular-shaped crystals range from 0.1 to three microns in diameter. The emulsion also contains a blend of color couplers (which form the color dyes in films) and other chemicals, that determine such imaging characteristics as contrast ratios, speed or sensitivity to light, granularity and color saturation.

What happens when light strikes a film frame is a combination of the aesthetic taste of the cinematographer, and pure science based on chemistry and physics. There are also random patterns of T-Grain crystals embedded in each frame of the emulsion. That is very similar to the way the human eye works. Light is a form of electro-magnetic energy that travels in a stream of particles called photons. When light reflects off a person or object, it sends a stream of photons through the lens. When a photon strikes a silver halide crystal, it causes a chemical reaction, which releases electrons that form latent images consisting of small clusters of silver specks. That cluster forms an analog representation of what was in front of the lens. This reaction also triggers the release of dyes that create colors. When a laboratory processes the negative, the latent images are amplified billions of times, which brings out nuances in contrast and subtleties in colors.

One of the characteristics that makes film a more flexible medium is that there must be a certain threshold of exposure to light before it begins forming latent images. That characteristic is what gives cinematographers the ability to manipulate light and shadows in ways that create lush blacks, pure white and literally hundreds of mid-tones in-between.

QUESTION: What happens when light strikes a CCD sensor?

SEHLIN: With a CCD, you have a rigid grid of pixels (picture elements) on the sensor. When a photon strikes the sensor, it registers the energy and converts it into numeric values, which are transported to some type of magnetic storage medium. Instead of a latent image you have a numeric representation, and instead of the random nature of the way the crystals are embedded in the emulsion, you have the exact same grid repeated.

QUESTION: What are the other differences between film and digital video?

SEHLIN: Color negative film has a much broader dynamic range of 1,000:1, compared to 100:1 with the best digital video systems. Those ratios describe the capacity for recording shades of tonality and colors between black and white.

QUESTION: How about 24P HD camera systems?

SEHLIN: The HD 24P cameras perform impressively in the shadow areas, but they lack the nuances that you get with film. Highlights tend to blow out in high contrast scenes.

QUESTION: Why is dynamic range important for filmmakers?

SEHLIN: The dynamic range of film is what gives cinematographers the creative freedom to create a sense of place and time, and to accentuate performances with light, colors, shadows and textures. These are some of the reasons why film feels organic to audiences. They tend to associate a film-look with fantasy and video with reality.

QUESTION: What about picture resolution?

SEHLIN: Typically, when you scan an image from a Super 16 mm frame you get a little more than a 2K file, which is slightly better than the resolution you can record with the latest generation of HD 24P cameras. Super 16 film retains the advantage in dynamic range, and the random placement of silver halide crystals, which contributes to the texture or look that audiences associate with fantasy. With film, cinematographers also have enormous creative freedom to interpret images by their choice of negative, the way they expose images, the color of light, the use of filters and diffusion, and how the film is handled by the lab. Cinematographers who shoot film also have tremendous freedom in postproduction. They can alter colors, contrast and other characteristics optically during "timing" sessions in a film lab. They can also scan and convert the processed negative to digital picture files, which can be manipulated to alter colors, contrast, sharpness and other characteristics of the images. Digital intermediate technology can extend the role of the cinematographer into the postproduction suite.

QUESTION: There have been some claims that high definition digital images are inherently sharper than film. Is this true?

SEHLIN: Thats' a complicated subject because a lot of things contribute to the perception of sharpness. One of the things that happens in electronic image capture is that the edges of objects and people are amplified, which makes the images seem sharper. That's actually a problem when a cinematographer wants to use selective focus to draw attention to something in the frame. As an alternative, if you want a totally sharp picture, you can begin with film and enhance the image during digital postproduction.

QUESTION: Some video vendors claim Super 16 film is too grainy for HD display.

SEHLIN: They are probably referring to a test conducted by engineers at one television network. An HD camera and 16 mm and Super 16 film cameras were locked down side-by-side at high noon on a sunny day. Several models were dressed in neutral colors and the people who did the testing chose to use the oldest and grainiest color negative film available. They also used what we considered to be an inferior telecine system to convert the film images to HD video format. The same engineers judged the results. Other tests have yielded different conclusions.

Furthermore, there have been significant advances in both film and telecine technologies. You have to judge for yourself, but the reality is that many producers are opting to produce programs for HD display in Super 16 format because they prefer the film-look.

QUESTION: There are claims that digital video needs less light for exposure.

SEHLIN: We believe that the "fastest" digital video cameras have a relative exposure index of 400 to 500. The Panavision HD lenses are said to be "slower" because the optics have to compensate for a relatively small two-thirds of an inch chip, which is about the size of a 16 mm film frame. Our film speeds range from 50 to 800. You can "boost" video and gain speed by sacrificing other imaging characteristics. You can also "push" film and trade speed for grain. The most important point is that cinematographers use light and shadows to amplify story points, so the idea that you can save time by not lighting is generally false.

QUESTION: Won't digital capture technology eventually catch up and pass film?

SEHLIN: Film is a moving target. Just a few months ago, Kodak introduced a new film, Kodak Vision2, that uses quantum leaps in the efficiency of light gathering to make tighter grain, more accurate color reproduction, and to make our film work better with digital scanners. As a result, Super 16 is now a significantly more versatile and easy-to-use medium than it was just a year ago. By leveraging advancements in digital scanners along with improvements to Super 16 mm film, budget-conscious filmmakers are creating a revolution in the industry. In many cases they are choosing the creative flexibility of the Super 16 option over 24P HD and other video systems. As much as we've improved film over the last 10 years, we feel the next 10 years will see even more impressive advances in technology.

We are also working on ways to use the strengths of digital technology in harmony with film's strengths and to put that power into the hands of filmmakers. We have demonstrated a prototype of a 2K/4K, high-speed scanner, along with Thomson, that is about five times faster than the next fastest scanner available today. The prototype scanner captures uncompressed image data at rates that were unheard of just a few years ago. Digital intermediate technology was merely an interesting concept just a few years ago. This new scanner will make digital intermediate technology much more affordable, and put the power of digital manipulation into the hands of many more filmmakers. We also have scientists and engineers working on computer processing techniques that will make 16 mm film look like 35 mm film, and 35 mm film look like 65 mm film. We can only imagine what large format films will look like using these processes. The key to all this is that there are more details on the original film than we can see with today's postproduction or display technologies. In the near future, digital systems will allow us to take advantage of the full measure of information in a frame of film.

QUESTION: What about archivability? Is that an issue?

SEHLIN: Today, film is by far the most reliable way to archive images. Black-and-white separations will last for up to 500 years, and color negative and intermediate stocks will last for hundreds of years, presuming they are stored correctly. Digital video is an improvement over analog video signals because it is ones and zeroes, but the storage medium is still either a magnetic tape or disk, which is comparatively volatile. Once a digital signal is gone, it's gone forever.

The other problem is obsolescence of formats. More than 75 video formats have been introduced since 1956, and even if the media survived, in many or most cases, there is no equipment for playback.

QUESTION: What does the future hold for film?

SEHLIN: The future, by definition, is still undefined. I believe film will remain the medium of choice for narrative, TV commercial and documentary producers, and that the Super 16 format will provide cost-effective options to HD video. Our experience is that most filmmakers embrace the tools that best serve their needs. I don't see that changing. The next few years are going to be a lot of fun, watching how the creative community uses the new Kodak Vision2 film and digital and hybrid tools. There are fundamental differences in the ways that film and digital images are recorded, perceived and stored and I don't see that changing. I read and hear all of the digital hype once again predicting the death of film, and I get a kick out of it because I know what the possibilities are for the future.

Dick Sehlin, chief technology officer and vice president of Kodak's Entertainment Imaging division

[Alex]

Wow, lots of good stuff.

I had a couple of reservations about a couple of points but otherwise it was pretty cool.

Glad to have read it.