THE TRADITIONAL LAB PROCESS
When a cinematic release is shot on film, whatever format chosen, typically 35mm, up until recently the process to the cinema has remained unchanged. After developing the camera negative, ‘rush prints’ are struck and viewed to make sure the crew captured the required action. This is to check there were no mishaps on the shoot. The production may have moved on several days by the time the rush print returns to the director. He or she may have to rely on someone else’s judgement that all has been captured. If a set has to be dismantled or the crew has moved location a re-shoot can be a costly affair. Where as ‘pick-ups’ can be a relatively minor shot filmed or recorded after the fact to augment footage already shot, costing less to the production.
An initial optical grade may be done on these rushes to ‘normalise’ the image for viewing and for editing purposes. These rushes are compiled into what we would recognise as the final film, known as the ‘work print’. The original negative is only then finally cut to this ‘work print’, which in itself is risky because if you make a mistake at this stage you will never be able to recover that cut frame, because you can never rejoin the cut invisibly. The work print essentially provides a cutting list.
Along the edge of each negative (also the work print) is a unique numbering, or keycode. Keycodes are unique to every piece of film. They are exposed into the raw stock at manufacture, which only become visibly during processing. Through these codes any individual frame can be identified within the production. Precise matching is made using a dual gang synchronizer, the work print running alongside the negative and visually comparing the action and these keycodes.
From this edited negative an inter positive (IP) is struck which is usually known as the safety copy, in case of damage to the original. Then an inter negative is struck which has no splices and has scene-to-scene grading corrections which can then be printed at high speed as a ‘one light’ which finally produces the print we view at the cinema.
It is useful to note that that these copies incur quality losses, so some detail of the original negative is lost, something that the Digital Intermediate process does not.
Colour Grading Lab Style
Film is shot under many conditions and differences in light. Film density in the raw film stock and exposure needs to be evened out to provide continuity between scenes back to back. They may have been shot days apart, on different locations or it may simply have become overcast part way through the days shoot. Even pick-ups months apart may all have a different hue, due to developing differences (the bath may be slightly different), different film stocks, lens or even a different filter can change the look. Negatives must be graded, sometimes referred to as colour timing or what we think of as colour correction.
Traditional optical lab work only allows the overall balance and density to be changed. Individual colours in an image using optical grading techniques cannot be fettled without affecting the entire image. Without explaining the full logistics of how an optical printer works, the basic idea is that the full white printing light is separated in to its individual red, green and blue components. Each of which is passed through a light valve to adjust the intensity before recombining the sources at the printing gate. These valves are opened and closed by fractional amounts, letting controlled amounts of light through which are known as printer settings or lights.
The printer lights are determined during the grading session before printing. A change in one single point of colour will be barely perceptible. If a negative is overexposed by one stop in the camera, it takes approximately +7 printer points to correct for this. Another point, which can cause confusion, grading corrections work in reverse of how you would think, adding two points green actually removes green. Confused – well it’s not easy and can be truly seen as an art form in its own right.
Grading ultimately cannot compensate for under-exposed negative, and some shadow detail will be lost however much the print is lightened – some may refer to this as becoming ‘milky’.
It should also be noted the tolerance in projection standards around the globe will affect the look anyway, due to the intensity of the light source. Even down to the glass separating the auditorium from the projection room will change the colourimetry!
Some labs use a computer to log these printer light changes on every scene, using the frame count cue (FCC) these trigger points, cue the mechanical changes in printer lights. However some still use punch paper tape that runs alongside the film, as the name suggest punched holes trigger these printer light changes, it sounds archaic but the system has proved extremely reliable.
Remember when we are watching theatrical releases at the cinema via an optical projector, even though the film has gone through a DI process, we are still reliant on the lab to produce the final print runs.
The move towards DI away from the optical process’ of old is pretty much the standard work flow now.. Most films you see in the cinema will have been through a Digital Lab. The process allows a far more creative, controlled, flexible and intricate style, compared to optical grading. Plus films are rarely viewed in a ‘film only’ environment now. Chains and many independent cinemas, with the help of the Arts Alliance (UK) have ivested to go digital. Films are delivered on a hard drive, which are ingested onto a server (large hard drive) or downloaded to the server and then projected as a high quality image via streaming media through a digital projector, thus alleviating the expensive option of going back to film. Although I suspect traditional deliverables will still demand an optical copy. This is a compelling argument in itself to take your production through the Digital Intermediate route.
At some point if the project is captured on film it needs to be scanned or telecined for multiple formats to be struck from this master. D-Cinema, High Definition, DVD and a myriad of television standards will be struck from the Digital Intermediate master timeline. Many labs now are offering the full DI route as well as the standard lab optical process' offering a 'one stop shop' under one roof.