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

The digital still-camera’s accessory “equipment”

The photographic industry offers a huge number of products to make the life of the photographer easy. In the following section, some tools are examined, not only to understand their potential and usefulness, but also to check their consequences for the reproduction of slides and lanterns.

 

6.1 “Obligatory” equipment: histogram, “live histogram” and clipping warning

A histogram is a graphic representation of all the light respectively colour values the sensor has captured. The left side of the diagram stands for no light or no colour, right for full light or full colour. A histogram shows how many pixels have the same values. If most of the graphic is near to the right side, the picture is overexposed, an overload on the left side signals an underexposure; a peak that touches one of the histogram’s extremities shows that values were “clipped”. This tool can be used after the take to make sure that the photograph is balanced (and also to decide whether it is better to take it again), or before to check whether parts of the motive will be over- or underexposed (and consequently change the light situation or the camera position). In the latter situation, the tool is titled “live histogram” (Striewisch 2009, p. 346). As a file saves each of the three primary colours separately (“colour channel”), each colour has its own histogram that demonstrates the captured light values (King 2017, p. 184). Some cameras have a “clipping warning”: those parts of the motif are marked on the display (viewfinder, LCD) which risk to be badly photographed due to insufficient or too much light.

 

6.2 “Obligatory” equipment: The anti-shake function and the tripod

To avoid blur, a camera-stand is extremely useful as the camera is fixed in a controllable position. However when the photographer has to travel to visit e.g. a collector, it is not practical. Two tools can substitute it: an in-built vibration reduction (aka anti-shake technology) if the photographer is experienced and has secure hands (for more see section “The digital still-camera and its components. 7. Camera in-built tools: the image stabilizer” in the photographic section) , and a tripod.

 

The tripod should be stable and therefore relatively heavy; light ones, which are nice to travel with, can be quite unsteady and transfer slight movements to the camera (e.g. when pressing the release button). When it is necessary to travel with light luggage, a folding or telescopic monopod can also be practical in case the camera has no “anti-shake” function. However, the monopod is only for trained operators with calm hands. For photographers with possibly trembling hands, the use of a tripod is obligatory to avoid blurring the image (Wunderer 2015, p. 24-25), especially in low-light situations which impose longer exposure times, for close-ups or if satisfying illumination s/he chooses to work deliberately with longer exposure times and smaller apertures.

 

6.3 “Nice to have” equipment: external flash and diffuser

Most digital still-cameras dispose of an integrated flash which can support the lamp set installed for the reproduction. This integrated light source is generally not so strong (otherwise it would abuse too much of the accumulator’s energy) and thus has a small radius (up to three meter according to Freeman 2004, p. 62, eight to ten metres according to Voigt 1973, p. 102). An integrated flash has the disadvantage that the flashing direction can only be varied vertically to make indirect light possible by sending it to the ceiling. Photographic experts generally advise to disconnect it and work with an external flash which assures higher flexibility. It can be fixed to the “hot shoe” (the metal socket on top of the camera) or used as a separate unit connected by a cable to the camera body. As electronic flashes are designed to send a “neutral” light imitating sunlight, they can help as additional light to avoid shadows on objects.

 

Some archives use the flash as additional (filling) light (see “Practical Recommendations by Anke Napp, archivist at Kunstgeschichtliches Seminar der Universität Hamburg“).  If the flash shines directly on the object it can produce an overexposed centre (too bright) and underexposed parts (too dark). Indirect flash-light, produced by its reflection from a wall, a ceiling or a reflecting surface placed not far from the object, avoids this effect. With a camera-stand this is not practicable. A diffuser put in front of the flash softens its intensity.

 

6.4 “Nice to have” equipment: lens extension systems with tubes and bellow

Objectives can be made longer with the help of rings (so-called extension tubes) and bellows (light-tight sleeves) which are put between the mount and the lens. Extensions can be helpful when the photographer has chosen to work with a fix focal lens and needs to take close-ups (enlargement of details on the slide). A number of cameras allow only a minimum distance of 80-90 cm between lens-system and object (called the “object distance”), when the device is brought nearer the object’s picture gets blurred. The use of a zoom would make it possible to approach the motive with its details still being in focus, however a zoom-lens generally “eats” light.

 

Extension tubes and bellows are an alternative: they extend the objective and thus its focal length. The focal length is the distance between the centre of the lens-system and the point inside the camera where all incoming light rays “meet” when the objective is set at “infinity”. The focal length is marked on the objective (e.g. 28mm (wide angle), 50mm (normal) etc.) and is invariable, except for zoom-lenses. “It is important in photography as it decides the scale of the image (the longer the focal length, the larger the image) and the extension of the camera (a longer focal length calls for a greater amount of camera extension).” (Focal 1973, p. 658) The last can be observed when a zoom-lens “zooms in”: the lens groups move forwards which makes the objective longer.

 

A bellow is a “collapsible and light-tight sleeve joining the lens to the back of the camera”, a “bellow extension” is an “adjustable extension unit […] intended for fitting on cameras with interchangeable lenses […]. Such a  unit provides a wide-range of focusing distances, especially for close-ups and macro-photography.” (Focus 1973, p. 99-100). To work with a bellow extension makes the camera more flexible: as the bellow is inserted between camera body and objective, the lens-system can be moved forward and backward. When the camera is fixed on a reproduction-stand, the frontal lens can approach the slide and withdraw from it, by this decrease or increase the angel of view, enlarge or narrow the depicted areas of the object and magnify details up to a certain point. Thanks to the extension a standard 50mm objective can turn into a “macro-lens” and make sharp images although the object distance may only be some centimetres. The same effect is created by adding one or more “extension tubes” or “extension rings” which are screwed into the mount (and also into each other), the objective is fixed on the other end.

 

However by adding tubes, rings or a bellow the depth of field is progressively reduced when the object is approached more closely. For a flat glass slide for a toy lantern this is of little importance, but when it comes to wooden framed pictures tests have to be done on the minimum distance before blurring starts. Also a ring or tube prolongs the way of the light through the objective and will slightly reduce its speed, so the more rings or the longer the tubes the more light is “eaten”.

 

Michael Langord (2008, p. 97) indicates the magnifying factor of a set of three different extension rings, made for a normal objective (50mm) and a full-frame sensor:

 

Number of rings Magnifying factor Distance to object
1 c. 0,15 – 0,25 times c. 35 – 25cm
2 0,25 – 0,3 25 – 20
1+2 0,3 – 0,5 20 – 16
3 0,4 – 0,6 16 – 14
1+3 0,6 – 0,7 14 – 12
2+3 0,7 – 0,8 12 – 11,2
1+2+3 0,8 – 0,9 11,2 – 10,5

 

6.5 “Nice to have” equipment: the external light meter

Today a light meter is part of every camera. DSLR cameras normally measure the light intensity “through the lens” (TTL), older devices had a photo-cell integrated into its front. However as photographic expert John Freeman (2004, p. 32-33) states, the in-built light meter in the digital camera only counts the light that is reflected from the object while a hand held separate light meter allows the measurement of the light emitted from the light bulb, rather than the quantity which really arrives on the slide. It is therefore informative about the “incidental” light of the scene. The internal light meter notes the darkest and the brightest elements in several segments (“multi-segment metering”) of the motive and tries to find an average “balanced” setting. Or it focusses on the centre of the object to be taken which can be in a broad (“centre-weighted metering”) or concentrated (“spot metering”) way. For slides, which are lying immobile under the same light source, spot measurement could be as productive as centre-weighted measurement for the three-dimensional lantern. A hand held light meter can be practical when the slide is heterogeneous in brightness: large black painted (or darkly coloured) areas in opposition to clear parts with strong light falling through. Here the in-built light meter may have difficulties determining the right aperture and exposure time to avoid over- or underexposure.

 

6.6 “Nice to have” equipment: the remote shutter release

When working in a low-light environment with longer exposure times, a release button which opens and closes heavily can cause vibrations even when the camera is placed on a camera-stand. The little movements blur the image and can create multiple contours. A remote shutter release could be a possible solution: the camera body can be controlled in a remote, hands-free way. There are mechanical ones using a long cable with a wire inside which pushes the release button; this cable release is to be handled speedily but without rushing (Horder 1958, p. 171). Photographic expert Friedrich-W. Voigt (1973, p. 80) recommends a long one which should form a little bow as a tense wire could produce reverberation. If the camera allows it, photographer Joe Hedgecoe (2004, p. 23) suggests using a remote control for a distant release. A third solution could be to use the automatic release. Tom Striewisch (2009, p. 75) suggests using it if the camera cannot work with a remote release. As the names state, the automatic and the remote shutter release open the diaphragm without hands touching the camera body. Some cameras have a “silent (shooting) shutter” which opens and closes smoothly (and without the loud “click-clack”) to avoid the shutter producing unwanted shock vibrations. It is necessary to test how well the system works and whether it can replace a remote shutter release.

 

6.7 “Nice to have” equipment: the software “High Dynamic Range”

 

Each camera is delivered with a CD-ROM containing a special software which it uses to communicate with the computer. The software is programmed to be compatible (or not) with older cameras of the same company and (certainly) not with those of other brands, it also becomes obsolete after some years as software producers go on with newer hardware models and the next (possibly not backward compatible) software version.

 

Nevertheless it may be interesting to look for special programs a company has to offer. The “High Dynamic Range” (HDR) function some cameras are equipped with is useful when a slide has strong (colour) contrasts which a camera may not be able to deal and may lead to clipping. “High dynamic range” means that the camera is capable of registering light values from very bright and extremely dark areas (thus both ends of the picture’s spectrum) and reproduce fine details. “Clipping” means that data will be lost in the concerned segments: the image will show just bright / white and dark / black areas, any details will be gone.

 

Contrast is essential: “Without contrast there could be no such thing as a visible image. A line in a photograph is visible only because it is either darker or lighter in tone than its ground. Every distinguishable part of the image is the result of a contrast in tone values.” (Focal 1973, p. 345) The human eye (in cooperation with the brain) is capable of seeing a high range of different light nuances. According to photographic expert Hugo Schöttle (1978, p. 133), with an unchanged iris opening the eye perceives a light-dark-contrast of 1:1000, when the eye has to adapt to different light conditions it even notices differences up to 1:300.000. It is highly variable and follows changes easily, and thus presents a “high dynamic range”. Compared to this, Schöttle states that a photographic picture has a very limited dynamic range (1:30) between the areas of minimum and maximum brightness of the taken scene, a negative and a diapositive film reproduce much more light values (1:1000).

 

Contrast is a delicate point in photography where it is defined as the relation between light and shadow: “high contrast” means strong brightness and deep blackness, thus hard separations between parts, “low contrast” is characterised by “smooth” transitions between sections, much grey, week light and dark parts. For slides both is possible: total transparent (white) and total dense (light-absorbing), strongly opposing areas mainly in painted and printed specimen, moderate variation in tones for photographed ones. A high contrast slide, e.g. a hand-painted sketch with black and red ink lines on a glass-plate that otherwise is empty (just pure glass), will be reproduced as an image with strong white (glass) spots, some red to reddish sections, some (very) black and also some dark grey parts (where the black lines are less absorbing) which makes it a picture with few variations and almost no intermediate tones. A low contrast slide, e.g. a black and white negative for a slide, will be depicted as “soft”: the reproduction will contain a (wide) range of different greys, but will have only few pure blacks or whites.

 

The difference between the brightest and blackest spots on the object is measured with the camera’s internal light meter which indicates its “brightness range” (Focal 1973, p. 345). The camera will then determine the ideal aperture of the diaphragm and an adequate exposure time. However, with strong differences between the clearest and darkest sections the camera may be unable to cope. Schöttle (1978, p. 152) gives an example: in a picture the clearest element would ideally be taken with a small diaphragm aperture of f22, the darkest section with a relatively wide open f5,6 (with the same exposure time). A difference of four stops has to be reproduced. As one stop doubles the amount of light than can pass the “threshold” (diaphragm), the brightest element is four times clearer than the darkest. The brightness range of the picture, or “contrast range” as Schöttle calls it, is 1:16 which is relatively slight, but the authors stresses than 1:1.000.000 can be reached.

 

A still-camera has a much lower tolerance to light differences than the eye. The device tends to ignore nuances in the darkest and the clearest part of the histogram (clipping). Its “dynamic range” describes best the values in the middle of the scale of light intensities. To repair this “incapacity”, the photographer takes several shots of the same scene, each time with another exposure time, but the same aperture in order not to change the depth of field. According to Chris Catcum (2014, p. 84), the HDR-software combines the images and fuses them into one offering the highest contrast range (a “full dynamic range”), including those values that were not represented before if only one single shot had been taken. This may help in reproducing slides that have different density parts: e.g. slides by the French company Lapierre with intense bright colours but “intertitles” missing blackness which makes them difficult to read; or the animal series by the British manufacture Carpenter & Wesley which show species designed with fine grey lines (sometimes are hardly recognisable), partly coloured in a small variety of mostly pale tones, surrounded by a deep black background. Their difference in brightness leads easily to under- and overexposure.

 

Nevertheless, a deontological question imposes itself, i.e. whether the result can be accepted as a 1:1 reproduction, as the HDR-software produces an average of several shots, taking the best light values from each. On the other hand, also the human eye adapts easily to differences in light intensity, so why accept this from the camera eye as well?

 

6.8 “Nice to have” equipment: display loupe, hood shade and control monitor

 

Many cameras work with LCDs on their back. Numerous Liquid Crystal Displays are not bright enough to reproduce the image to be taken in bright daylight or similar conditions. Instead it works like a mirror and reflects the photographer’s face. To shield it from unwanted illumination several companies have created detachable sunshade camera hoods for the rear monitor. A hood can also be practical for the objective. It protects from stray light which may create flare on the objective and will be (slightly) visible on the digital image. It is standards in the film business to work with an external screen to control the framing and other parameters of the image. Some cameras allow to attach a external (field) monitor which could be used to check the horizontal position of the slide, the brightness of the reproduction, the presence of undesired reflection or the sharpness of the image (by zooming in). For the verification of the colour the camera own histogram is a better advisor.

 

6.9 “Nice to have” equipment: grey scale and colour checker

This equipment has been presented in section “Additional scanning tools” in the scanning section.