How to digitise slides. Recommendations and working lists for the reproduction of a very special artefact

What can be expected from the digitisation of a slide – a deontological reflection

Slides are historic artefacts, produced between 50 and 300 plus years ago. Over this time span techniques and methods have undergone considerable change. Their digital reproduction is limited by the modern technology’s efficiency in dealing with the material’s specific features, e.g. differences in the aesthetic look of a (hand) painted / printed / photographed / “copied” slides, the quality of the used dyes / printing inks / gelatine image, their translucency / opacity (density) and their saturation grades, the physical state of the slide etc. (see section “Technical components of digitising an artefact, 2. Colour capturing and Bayer mask” in the technical section).  The Swiss researchers of the DIASTOR project speak of “material-scanner interaction” (Flueckiger 2016, p. 110) and have analysed obstacles in relation to both. In the following section the focus lies on the “material-camera interaction” as well as other factors that can make the reproduction problematic.


5.1 Certain factors which influence the reproduction of slides


5.1.1 The broader context of the digitisation procedure


The conditions of the working space in which the shooting is done, the interference of disturbing factors from outside the working space, the time that can be spent on reproduction per slide, can have consequences. But at least they are (relatively) controllable by the archive unlike other factors.


5.1.2 The physical and mechanical constraints of the digital still-camera


The visualising potential of the camera itself depends on the technical skills of the manufacturer. It is determined by


  1. the quality of the hardware, e.g. the focal length and speed of the objective, the absence / presence of distortions and other aberrations due to flaws in the lens-system, the size and resolution of the sensor, the technique of the (electronic) viewfinder to facilitate an evaluation of the image’s qualities before pushing the release button, the display’s capacity to visualize the taken image adequately on the screen and allow (with the help of tools e.g. a histogram) one to check the colour correctness, brightness and sharpness;


  1. the potential of the electronic: the sensitivity to light of the photo-diodes, the kind of sensors employed (see section “Technical components of digitising an artefact, 1. The image sensor” in the technical section), the speed to send files to the storage system (bitrate), the size and storage capacity of the memory card to “digest” big amounts of pixels. The processor plays a crucial role as it manipulates the data according to pre-programmed settings, an automatism which often cannot be switched off. As photographic teacher Dennis P. Curtin (2011, p. 21) states: “Each time you take a picture, millions of calculations are made in just a few seconds. It is these calculations that make it possible for the camera to interpolate, preview, capture, compress, filter, store, transfer, and display the image. All of these calculations are performed in the camera by an image processor that’s similar to the one in your desktop computer, but dedicated to this single task. How well your processor performs its functions is critical to the quality of your images but it is hard to evaluate advertising claims about these devices. To most of us these processors are mysterious black boxes about which advertisers can say anything they want. The proof is in the pictures.” And he adds (p. 21): “When a camera company programs its processors its goal is not to exactly reproduce a scene’s colors. Instead, using a process called color rendering, its goal is to create what the programmers believe will be a pleasing reproduction. Frequently the contrast and color saturation are boosted, especially in the midtones and specular highlights are compressed for printing and viewing on typical displays.”
  2. the capacity of the software: the range of possible file formats, the utility of the pre-selected metadata standards, the proposed compression rate (see section “Technical components of digitising an artefact, 5. The compression” in the technical section);
  3. the flexibility of the camera: the chance to choose e.g. to work automatically or manual.


5.1.3 The human factor – parameters set by the photographer


The quality of the reproduction aka the technique’s efficiency depends on the way the camera and the illumination are handled. It is a result of several factors:


  1. the capacities and motivation of the staff, the personal predisposition of each: the caring /careless way to deal with the work, the respect towards the targets of the digitising project and the general rules of the archive, the creativity to find solutions for unforeseen difficulties; in post production: his/her eyes’ sensibility for colours and differences in brightness when correcting colour and balancing light; the staff member’s experience to handle material with deficiencies (e.g. faded colours, under-/over-exposed negatives) and to understand whether s/he is confronted with a flaw (due to careless treatment or inadequate climate conditions) or an original visual aspect produced by the devices technical characteristics;


  1. the quality and efficiency of the training by (post production) companies or the archive itself;


  1. the standards and traditions set by certain “schools”: in photography e.g. the predilection for certain camera setting or tools which may not be favourable to capture the particular qualities of a slide; in post production (restoration) deontological formulae which are understood as “laws” and executed by “followers” without questioning their suitability.


5.2 The limits of digital photography


The factors discussed above, as well as others – some controllable, some not – influence the making of the digital photograph. We can wish to capture a slide in all its light and colour values – the ideal “1:1 reproduction” –, the received image will always be different to “what the photographer saw”. There are several reasons. As photographic expert Tom Striewisch (2009, p. 284) writes: “The photograph does not reproduce the reality, it represents an abstraction of the reality.” A constellation that the photographer can see, hear, feel, smell is reduced by photography to a two-dimensional touchable surface (the negative or positive) or a two-dimensional simulation on a screen or monitor. It is evident that no camera will ever be able to reproduce a slide and make that the image “feel” as authentic as the original. Photographers have experienced this for almost 200 years, and so with digital picture nothing has changed. To recall the analogy to negative film stock: a digital reproduction is nothing else than a “latent image” of the artefact which would not become visible without helping activities such as “developing” (AD-transformation), “rinsing” (processor) and “fixing” (monitor).


Also one must know that many sensors react with higher sensitivity to certain light bands which the human eye has difficulties perceiving. The human eye cannot see dark red at the low-frequency end of the spectrum (normally humans can only feel infra-red light as warmth), and, ultra-violet light at the high-frequency end, correctly. UV rays can give an invisible tint to certain colours. However, photo-diodes can register and reveal them on the reproduction. Also fluorescent UV light, to which the eye is relatively insensitive, can influence or even change the appearance of an object, which has absorbed the light rays, adds them as short (blue) waves to the light bands it emits (Eastman 1992, p. 52-53). Dark red details on the slide may also look different when photographed.  Therefore, the photographed colours may not be identical to those perceived on the slide by the human eye.


As the digital picture is virtual, it has to be made “concrete” on a screen (monitor, camera display etc.). The “transmitter” also interferes considerably on how the reproduction looks. Screen expert Hubert Nasse (2009, p. 6) gives an example: “The camera has exactly 2832 pixels (2×1416) on 24 mm of image height. The monitor would have to have at least as many pixels [as the camera sensor] to be able to display this image information free of losses. However we will usually have to be satisfied with a lesser monitor performance, e.g. 1600 x 1200 pixels. The monitor can therefore only display parts of the full image without losses. If one runs Photoshop on a monitor with 1200 pixels in the vertical direction, some of these pixels are taken up by the menu bars and the net number of pixels seen is, for example, only 1036 pixels.” He insists on the distance between viewer and monitor plays as well as on the viewing angle which both influence considerably the perception of the displayed image. Last, but not least, there is the “subjectively perceived image quality” as the human “judgment [sic] will vary depending on the motif viewed” (Nasse 2009, p. 8).


Due to these limitations, it is an illusion to believe that a digital simulation could contain all data exactly as present on the original slide. In this context, one could ask a deontological question: if “what the photographer sees” when s/he has the original in hand – if s/he is not colour blind or has other visual defects – can not be reproduced and never experienced like the object itself, why bother? Why be true to the object? Why not “cheat” as the human eye and brain also do (see “White balance adjustment and automatic white tracking” in the photographic section),  or the photography and scanner industry when they manipulate the user’s senses (see section “Technical components of digitising an artefact, 2. Colour capturing and Bayer mask” in the technical section)?  It would certainly be convenient for some to overlook inadequate light and colour values on digital photographs as it is time-consuming to find solutions for this kind of difficulties.


However, the  restrictions should not be a motivation to leave aside all efforts to get the best out of the camera and its accessories. If the photographer does not try to respect the object’s measurable characteristics, if s/he does not try to recreate them as closely as possible with the help of tools (e.g. colour and light filters, flashes and diffusers, histograms and light meters, a careful selection of light sources etc.), if her/his skills and talents are not challenged to get the best out of the still-camera’s potential, who else will do it? And is it not the task of an archive to keep objects from the past accessible and alive for present and future generations in their authentic form? The investment of money, time and skill should lead to the most faithful reproduction of slides possible as they are references to historic moments in time, to a form of entertainment and education which was extremely popular for several decennials, and to a production process that has (almost) fallen into oblivion.