Tutorial 2 – Film basics

The film process is designed for capturing images from scenes that will be edited and copied for eventual projection onto hundreds or thousands of big cinema screens. This has been in operation for over 100 years and so has been developed and refined to a very high degree to precisely meet these objectives. The film stocks used in cameras have a characteristic that allows them to capture a very wide range of scene brightness with good color saturation to provide wide latitude for color correction after processing. Intermediate stocks used to copy the one original negative are designed to be as faithful to the original as possible. Print stocks provide the high contrast needed to produce a bright and good contrast image on the projector screen to overcome the background illumination in the theatre.

Television is different in many ways. For example, the results are always instantly viewable and are delivered, sometimes live, to millions of smaller screens. The sensors used in video cameras do not presently have the wide dynamic range of film and so shooting with them has to be more carefully controlled as the ability to correct exposure faults later is more restricted. Also the viewing conditions for video are different from cinema. Not many of us sit in a darkened room to watch television, so the images need to be brighter and more contrasted than for film.

The three different basic types of film stock used – camera negative, intermediate and print – each have very specific jobs. Camera negative records as much detail as possible from the original scene, both spatially and in range of light to make that original detail eventually available on a multitude of internegatives from which are produced thousands of release prints for projection.

The Film Lab
Between the camera negative and the print there are normally two intermediate stages: Interpositive and Internegative. At each point more copies are made so that there is a large number of internegatives from which to make a much larger number of release prints. The object of these intermediate stages is purely to increase the number of negatives to print. This is because the precious and unique camera negative would be effectively destroyed with so much handling. The intermediate materials, interpositive and internegative, are exactly the same and designed to make, as nearly as possible, exact copies for each stage (with each being the negative of the previous stage). For this requirement the material has a gamma of 1.

But the release print is not just a film representation of the shot scenes: editing, visual effects, and grading – not to mention audio work – must take place in between. This mainly works in parallel with the film processing path – partly to reduce handling the negative.

The camera negative is printed to make the rush prints which provide the first viewing of the shot material. Note that this will be at least several hours after the shoot so hopefully all the good takes came out well! The first edit decisions about what footage is actually required are made from the rush prints with the aid of offline editing.

Film basics

The negative cutter has the responsibility for cutting the unique footage according to the scene list. Initial grading is applied as the cut negative is transferred to interpositive. Should there be any further need of grading, instructions for this are sent with the internegatives to the print production labs. Any need of dissolves rather than cuts, or more complex visual effects, will require work from the optical printer or, these days, a digital film effects workstation.

Grading or Timing
Grading is the process of applying a primary color correction to the film copying process. The original camera negative may contain lighting changes which will mean that scenes shot on different days or times during the day need to look the same but simply do not. By effectively controlling the color of the light used to copy the negative to one of the intermediate stages these errors can be much reduced to produce a scene-to-scene match. Grading is carried out on a special system equipped with a video monitor displaying the current frame from the negative loaded onto it. Three controls provide settings of the red, green and blue ‘printer’ light values that adjust the amount of each of the three lights used to image the frame. These adjustments allow the operator to balance the color and brightness of the scenes in the movie.

This results in a table of corrections linked to the edge code of the original negative. This table is used to control the optical printer making the copy. Most processing laboratories subscribe to a standard definition of the settings but this does not mean that settings defined at one processing lab can be used at another. The photochemical process is very complex and individual labs will vary, however they all aim toward a standard. The ‘neutral’ value for RGB printer lights is represented typically as between 25, 25, 25 and 27, 27, 27 – depending on which lab is used. To print an overexposed negative will require higher values, and an underexposed negative lower values. A change of one in the value represents one 12th of a stop adjustment in exposure. Differential adjustment of the values provides basic color correction.

This analog process is now often replaced by a digital process known as Digital Intermediate (DI).

 See also: Digital Intermediate