Slide production process – how is it done?

One of the questions sometimes asked is how do we convert digital data to film?

No, we don’t photograph a print-out or photograph a computer screen!

We use special film recorders and the process is very similar to that used by the cinema industry where an entirely digital movie, like Shrek for instance, is transferred to 35mm movie film for projection in the theatre.

We have been producing slides (diapositives or simply dias to our continental friends) for many, many years and currently run several film recorders. These boxes of tricks expose sequentially with red, green and blue light onto 35mm transparency film (plus colour negative, b/w negative film and larger film formats) and are capable of recording the most vibrant and yet most subtle colours in very high resolution. We currently use Kodak film processed in own laboratory using 6 bath E6 chemistry, giving us both the speed and quality we and our customers insist on.

So how does digital data get converted to analogue photographic film?
The photos you take on your digital camera aren’t colour ‘images’ at all, just a sequence of number values that describe the degree of lightness or darkness in each of red, green and blue, and are described as pixels. If you have ever looked at the ‘channels’ panel in Photoshop you can see how these can be thought of as three monochrome images, each representing red, green and blue components. When you view an image on a computer screen these values are assigned to the red, green and blue pixels that sit side by side each other. Look very close and you will see red, green and blue dots of varying brightness. Move further away and you are now unable to see the small dots and it gets blurred together and gives the illusion of a colourful image.

OK, so that’s the digital part. It so happens that photographic colour film is designed and manufactured with layers that are created to react to red, green and blue light by use of dyes in the layers that act as filters (the filter dyes are removed in processing). That’s handy. So if we expose the digital ‘red’ data through a red filter, ‘blue’ data through a blue filter and ‘green’ data through a green filter then we will end up with all the colour values of the digital data exposed onto the appropriate layers of the photographic film.

The film recorder comprises an instrument camera with lens. The lens is a very sharp, flat field, fixed focus, fixed aperture, non-interchangeable macro lens. The camera has a capping shutter that simply opens at the beginning of the exposure sequence and closes at the end, and has a motorised film advance/film rewind mechanism and that’s about it.

The exposure is made by an electron beam hitting a phosphor screen making a tiny point of light. This point of white light sweeps across the film in 2,732 discrete horizontal steps from top to bottom of the film frame. The beam intensity is modulated according to the value of the digital data it is representing – that is to say it gets brighter and dimmer as it sweeps across the film. As this is a continuously variable intensity of light – analogue – we lose the dot-by-dot nature of digital media, and in that one sweep it takes in the 4,096 pixel values of the width of the digital image. The beam drops down a step and repeats the process a further 2,731 times.

So we can see why there are no ‘pixels’ visible on the film in the horizontal direction, but why don’t we see the 2,732 discrete horizontal lines? That is due to the penumbra of the flying spot of light – it isn’t a hard spot but slightly diffuse. The softness of the edge overlaps the previous scanned line, blending everything together. The result is that the image on film is completely devoid of it’s digital dots and now smooth and analogue in nature.

This exposing (or ‘painting’ in technical terms) is performed three times. The scanning dot is white, and would result in a monochrome image, so the film recorder has a filter turret that allows the red, green and blue digital channels of the digital colour image to be sequentially exposed via red, green and blue filters, thus exposing each of the film’s red, green and blue sensitive layers in turn.

Once the 36 frames are exposed the film is removed and processed in conventional E-6 chemistry (an established 40 year old process), in the same way that all photographic colour transparency films are processed, magically changing those red, green and blue exposed layers into cyan, magenta and yellow dyes layers, transforming mundane bits of digital data into glorious ‘old school’ photographs that are indistinguishable from slides shot in conventional film cameras.