I – Is it possible to compare the quality of analogue film image with that of digital image?

II – A necessary collaboration between archivists and operators of digital processes.

Digital treatment of film image were originally designed for production special effects. Specific solutions are now available for film archives. Some are still financially out of reach for many archives.

From the beginning, mostly because nobody knew exactly how digital processes worked, film archives have been reluctant to use them. It seems that we have now come to a more reasonable approach of these tools, though there are still many questions on the effects of digital treatments.

As an attempt to make sure that none of the original characteristics are lost, studies are made to establish relations between the original film image and the image resulting of a digital process.

1 – As some of you may remember from last year Seminar in Budapest, we have seen that a digital image is characterised by :

pixels (picture elements)
each pixel having a tonal value (black, white, shades of grey or colour)
horizontal and vertical measurements of an image are expressed in pixel dimensions
a digital camera will have pixel dimensions expressed as the number of pixels horizontally and vertically, that define its resolution (1024x768 ...)

resolution : resolution is the ability to distinguish fine spatial detail. Dpi (dots per inch) or ppi (pixels per inch) are synonymous terms to express resolution.

colour is characterised by the number of tones (greyscale or colour) : bit depth (number of bits used to define each pixel. A colour image is represented by a bit depth ranging from 8 to 24 (or higher), that is 8 red, 8 green, 8 blue.

compression of the file size. Compression is used to reduce image file size for storage, processing, transmission.

2 – An analogue film image is characterised by :

photochemical transformation : light transforms silver halide into metal silver (Ag). Grains form the image. Dye clouds around grains to form image.

film layers :
Base (nitrate, acetate, polyester ...)
Sometimes a dark–coloured intermediate anti–halation layer
Emulsion layer (BW : Ag Br in gelatine, Colour : separate dye layers)
sensitometry : describes how film density relates to exposure. Sensitometric evaluation takes place under controlled and specified conditions.
spectral sensitivity : describes the degree of photographic response to radiation of various wavelengths.

resolution : the common measure used for film.

3 – Conversion of analogue image to digital is characterised by the scanning and the shooting process, that is : the file size, storage space, capacity of work station and time allowed to operate the complete chain of process.

2K or 4K ? These numbers correspond to 2048 or 4096 dots per frame.

Many film archives are trying to find out ways to compare digital film image resolution with analogue film image resolution in order to determine which level of scanning /shooting is requested to maintain the characteristics of the original image.

But its seems that resolution measurements are not objective.

Dan Rosen from Kodak Cinesite conducted a test showing that they are subjectively determined from resolution bar charts : a series of black and white bars, taping closer and closer together, are shown to an audience. Subjects are asked to indicate where the individuals bars blur into a grey patch. And it appeared that the results are heavily dependent on contrast.

It also seems that resolution is only slightly dependent on development and that maximum resolution is obtained at intermediate exposures.

Therefore, objective resolution measurement could be achieved through the measures of the response of an optical system to a modulation. In this case resolution measurements are dependent on :

illumination levels
overall contrast

Measurements must take into account the total imaging system :

Film
lens
printer
scanner

Film history is made of compromises :

grain against speed
BW looks grainier than colour

early motion pictures lenses were good but not great (for example Zeiss are considered great optics but it seems that their diffraction affects the image quality).

Last but not least : what do you see ?

what you see depends on the human eye

the very best eyes can see down to 5% contrast, but who can ?

4 – Conclusion

For the reasons explained and because motion pictures are shot under a variety of conditions :

differing lights (different contrasts)
various quality lens systems

the cinematographer visual acutance

All this means :
that the data capacity of film is very large ... in theory !

optimum resolution can only be realised under ideal, perfect conditions in an optical lab

resolution has not much to do with the human eye, lenses or practical lighting

As an introduction to Thomas Bakels's presentation

The use of digital techniques must be considered as inevitable.

Digital restoration techniques are expensive, but costs are lowering ...

Current digital treatments are not perfect but can be improved.

Digital restoration is a solution for major masterpieces where traditional treatments cannot achieve good results. Digital film restoration is the future.

Please, consider the history of film technology. Can we say that film directors were happy with :

film graininess

splices "blops"

the first Technicolor systems where blue colour could not be obtained

...

Therefore, should archivists say no to the possibility of digitally restoring an essential sequence of a film ? Should archivists be more restrictive than filmmakers ? Grading a restored film or a safeguard intermediate without having the original grading chart is accepted.

In his presentation on Metropolis restoration work, Thomas Bakel will show you some examples of unwanted effects that are the results of the state of the art of the restoration softwares. These imperfections must not hide the positive results. They only imply that archivists must work in close collaboration with the operators.

Ó Richard Billeaud – RBC Conseil 2003

Richard Billeaud: Former film producer and manager of one of the major French post–production facility, now free lance consultant specialised in audio–visual, film and sound production, post–production, preservation and restoration, in relation with the International Federations of Film (FIAF), Television (FIAT) and Sound (IASA). Member of AMIA (Association of Moving Image Archivists), FIAF, BKSTS (British Kinematograph, Sound and Television Society), SMPTE and CST (Commission Supérieure Technique de l'Image et du Son). Member of the GAMMA group (European Film Archives and Laboratories Research Group). Teacher since 1971 (educational and vocational training – IDHEC, Ecole Nationale Louis Lumiere, Paris III Censier, INA ...)