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

The potential of digital still-cameras and sensors

Digital photography is a big business. Today many different camera types are on the market. Therefore, before purchasing a still-camera it is necessary to decide the (main) purpose the camera will be used for. This decision determines which qualities are needed and which functions the apparatus has to have to fulfil them. For the following short overview numerous photographic books and manuals were read; in addition recommendations by experts in photographic shops and on the internet were collected. Professional medium format and large size cameras are not included. The price is not indicated either as it varies from country to country and changes quickly (although it is for most archives an essential selection parameter). Not all types are totally adequate when it comes to reproducing slides, nevertheless all categories are mentioned to keep as neutral as possible. However, to be clear about that point, the working group has a certain predilection for digital single lens reflex cameras.

 

3.1 Advantages and disadvantages of digital still-cameras

 

Before the camera types are examined, here some explanations: “parallax error” signifies that the object as seen through the viewfinder and its reproduction on the sensor differ slightly. (For more see “Direct and indirect viewfinders and their potential” in the photographic section.)  “Half automatic” means that the decision on the technical parameters for the next shot will be “shared” with the instrument, e.g. aperture of the diaphragm is chosen by the photographer and the corresponding shutter speed is determined by the camera. The “Electronic Viewfinder” (EVF) is part of the camera body. It shows the slide as it is placed in front of the objective with the help of a mirror-system which reflects its image onto an in-built screen. In many cameras the EVF combines the image of the object with selected information on e.g. ISO value, exposure time, frame size. “Through-The-Lens” metering (TTL) means that the intensity of the light reflected from the slide is measured by a sensor which receives the light rays through the objective, and not, as was earlier often the case, by a integrated photo-cell or an external hand-held light-meter (Eibelshäuser 2005, p. 124). Every electronic device produces noise, as does a digital camera: photo-diodes on the sensor send weak electric charges to their neighbours which were not hit by the light beam; these charges are reproduced in the image as if light had touched them, too. Noise can be intensified when the sensor is getting warm, therefore accumulator and back screen of a camera should be as far away form the sensor as possible (Striewisch 2009, p. 43-45). (For more on noise see “Some reflections on photographing techniques and shooting parameters. 4.2.1 White noise” in the photographic section.)

 

Camera type 1: High-end Compact Camera

This camera type has an extremely small “compact” body which makes it attractive when travelling.

Capacities: a simple standard compact camera (also known as “point-and-shoot camera”) is equipped with a zoom lens which folds into the camera when turning the power off; a zoom compact camera has a better zoom; a “high end compact camera” is their luxury and much pricier sibling, with an average sensor capacity of 20 MP (the range, as found on the internet, lies between 12 and 25 MP, one had even 42 MP) it can be used with manual, automatic and half automatic control. Generally, these cameras work with a back screen (no EVF) and deliver data in RAW and JPEG.

Size of sensor: e.g. 12,8×9,6mm (Sony), 18,7x14mm (Canon, “high-end compact”), 23,6×15,6mm (Nikon, “ultra high-end compact”).  (https://newatlas.com/camera-sensor-size-guide/26684/)

Additional equipment: built-in flash.

Potential: small, light body, easy to handle, ready to shoot, much cheaper than a DSLR.

Challenges: the lens is fixed, thus not interchangeable. The sensor is small and reduces the camera’s usability. Compact cameras are normally preprogrammed and automatically manipulate the image, but it seems that the integrity of the RAW file is respected with high-end versions of this camera type (Matthai 2008, p. 51). The interference can “include in-camera photo editing and special effects such as red-eye removal, image enhancement, picture borders, stitching together panoramas, removing blur caused by camera shake, and much more : […] The processed images can be so distinctive that it’s possible for some people to tell when an image was taken with a Canon or Nikon [DSLR] camera.“ (Curtain 2011, p. 21)

Usability for slide reproduction: the high end versions with a greater creative versatility may be convenient for small size reproductions to be used on the internet. As JPEG files are immediately “optimised” by the processor they may not need post production. As its sensor is rather small, the file cannot be used as source for a large copy. Compact cameras should not be used when long-term storage of images is planned or a true reproduction of the slide’s historic properties is the target.

 

Camera type 2: Compact System Camera (CSC) / Mirrorless (System) Camera (MSC) / Mirrorless Interchangeable Lens Camera (MILC) / Digital Single Lens Mirrorless Camera (DSLM)

With this camera type components (such as the objective) can be changed which makes it flexible.

Capacities: the camera is equipped with a zoom lens, works with manual and automatic control and delivers data in Raw and JPEG. Some have an optical or an electronic viewfinder to examine the object directly.

Size of sensor: the light-sensitive zone comes in variable sizes, from 6,17×4,55mm (Pentax) and 12,8×9,6mm (Nikon) to Four-Thirds (17,3x13mm, Lumix, Olympus) and APS-C (22,2×14,8mm, Canon) or even full-frame (36x24mm, Leica).  (https://newatlas.com/camera-sensor-size-guide/26684/)

Additional equipment: CSCs often come with an integrated or optional electronic viewfinder (why this camera type is also called Electronic Viewfinder Interchangeable Lens Camera (EVIL)); the EVF combines the “look through the objective” with histograms and other tools visible on the display. Not all CSCs dispose of an in-build flash.

Potential: CSCs have a light body and are easy to handle; thanks to their EVF objects in dark and nightly spaces are better recognisable (Striewisch 2009, p. 28).

Challenges: the LCD-screen on the camera’s back is not high resolution (Striewisch 2009, p. 29). Those with optical viewfinders show a parallax error which has to be taken into account when framing the object. The lens is not interchangeable. According to Wunderer (2015, p. 94) compact cameras electronically repair the consequences of trembling hands as they do not have a mechanic anti-shake. Good CSCs can be quite expensive.

Usability for slide reproduction: those equipped with a bigger light-sensitive zone can be suitable for reproducing slides as their sensors can combine a higher resolution with larger photo-diode surfaces. The take should be done in RAW for a more sustainable result.

 

Camera type 3: Bridge camera / hybrid camera / super-zoom camera

This camera type unites the characteristics of a DSLR with those of a compact camera which makes it a “hybrid camera”, also called a “bridge camera” as it “bridges” between the two systems.

Capacities: a bridge camera has a zoom lens ranging from wide angle to “super zoom” and works with manual and automatic control. Sometimes it is also called “prosumer camera” to mark its position between a professional and a consumer model.

Size of sensor: rather small, Four-Thirds and smaller (with only few exception).

Additional equipment: most have now a video function and allow filming in Full-HD-quality; generally, they benefit from a powerful zoom coming with an image stabilizer; an EVF on top of the body is optional.

Potential: it is operated similar to a DSLR as it has a hand-grip; many have a tilt screen and often an electronic viewfinder. As the characteristic mirror-pentraprism-system of a DSLR is missing, its body is lighter and smaller which brings the lens-system closer to the sensor. Generally, the shutter releases smoothly and noiselessly. The use of the electronic viewfinder or the LCD to take a picture consumes much electric power which reduces the running time of the batteries.

Challenges: as the sensor is closer to the back monitor, its screen’s warmth risks to produce unwanted noise. It seems that the relatively small sensors need amplification to reach the standard light sensitivity of ISO 100 (Wunderer 2015, p. 111) which normally results in noise.

Usability for slide reproduction: the objective cannot be changed, but the zoom has a high magnifying potential (from normal tele to wide angle, but no macro-modus) which allows the photography of both small details (e.g. manufacturer’s logos) and a voluminous optical lantern. It seems that the EVFs of bridge cameras have a lesser resolution than those of DSLR cameras.

 

Camera type 4: Digital single lens reflex camera (DSLR)

The DSLR camera recalls in many ways its analogue predecessor.

Capacities: a DSLR can be used with a wide range of interchangeable objectives, works with manual and automatic control and delivers data in Raw and JPEG. It disposes of an optical TTL-viewfinder; its display combines the image “viewed through the lens” with information on its photographic parameters. Generally, this camera type is equipped with an autofocus (AF) using nine up to 170  measure points for contrast as well as for phase detection.

Size of sensor: sensors in DSRL cameras can have different sizes; the more expensive cameras, and all those which are made for professionals, dispose of a “full format” sensor (36x24mm). More consumer oriented models have a smaller one, commonly called Advanced Photo System Type C (APS-C), e.g. 22,2×14,8mm (Canon) or 23,6×15,6mm (Nikon, Pentax, Sony).  (https://newatlas.com/camera-sensor-size-guide/26684/) Others use (Micro) Four-Thirds (17,3x13mm), APS-H (27,9×18,6mm) and Foveon X3 (20,7 x 13,8mm).

Additional equipment: these still-cameras are mostly sold with an integrated flash, more flash-lights can be added; they often work with a movable LCD-monitor at the back. Some models allow to adapt the ocular of the optical viewer to bad eyes (“dioptric correction”). Professional types have an additional LC-display on the body’s top. Some DSLR cameras are equipped with a video function and can produce still and moving images; when working in the video mode, the video image can be sent via a HDMI mini-micro cable (High Definition Multimedia Interface) directly onto a TV screen.

Potential: as with all digital cameras, the result can be controlled immediately. Once the picture is downloaded it can be reworked instantly and communicated via the internet. Changeable objectives for this camera type allow a flexible approach; the objectives are made by well-known producers such as Zeiss, Leica and Schneider-Kreuznach (Langford 2008, p. 112).

Challenges: the body is rather heavy and voluminous; the shutter opens and closes loudly and with some vibrations when not equipped with a “silent shutter”; the LCD at the back is difficult to read in bright light; to use all facets of a DSLR is much more complex than with other camera types (Langford 2008, p. 113). Viewfinders often show only 95% of what the objective really covers; this type of camera is very popular and has the shortest selling period of all cameras between the current model and the next one (Wunderer 2015, p. 32) which means that the bought camera is quickly “old”. Some photographic experts complain that the vision on the item is interrupted as the mirror-system has to be set aside while the shooting happens, but with the taking of (immobile) slides this should not be a problem.

Usability for slide reproduction: this type of camera is highly flexible and can be used with accessories such as filters, bellows and additional flashes; it can be manipulated manually to prevent, to a large extent, that the processor “optimises” the results; it seems that a DSRL camera can deliver RAW-files (almost) free from unwanted interference. As camera and lens-system are rather expensive, it is advisable to check the kind of objective(s) and accessories (sold in the same package) before deciding which model should be acquired.

 

Practical hints given by photographers

When buying a still-camera photographic expert Tom Striewisch (2009, p. 335-346) recommends paying attention to the following parameters: 1. pixels: the more pixels on a sensor, the smaller each pixel and the less light it can capture; 2. colour depth: the higher, the better; 3. contrast: the higher the colour depth, the better the contrast; 4. file formats: RAW is a must, JPEG and TIFF just to be used for post production; 5. sensor size: full-frame or bigger; 6. viewfinder: the better its quality, the more accurate the resulting image; 7. objective: a high quality lens-system is a must; 8. accessory: histogram, clipping warning, automatic exposure bracketing (AEB) are nice to have.

 

Elizabeth Halford (2010) also recommends the Automatic Exposure Bracketing: the AEB is “[…] a function most DSLRs have to take three photos with only one click of the shutter, each with different exposures. The result will be one photo a bit too bright, one just right (depending on which part you’re looking to expose properly) and one a bit darker.”   (https://digital-photography-school.com/bracketing-what-is-it-and-what-to-do-with-the-images/)

 

Photography teacher Dennis P. Curtin (2011, p. 15) remarks that smaller sensors with a high number of megapixels produce more noise and “require better, more expensive lenses, especially for wide-angle coverage”.  (www.photocourse.com/itext/pixels/pixels1.pdf )

 

The Benchmarking Art Images Interchange Cycles. Final Report 2011 reveals that one of the key results of the study on digitally photographing art work was the way the still-camera was selected: “It was interesting to note, but not surprising, that quantitative metrics for color and spatial quality often were not the primary selection criteria for the purchase of a camera system. Subjective criteria, word-of-mouth and technical support were viewed as more important. This research project showed that it is possible to develop a single experimental procedure to evaluate the objective quality of a camera system.“ (Berns, Frey 2005, p. 55) As to the photographer, the institutions interestingly had a precise vision: “The cultural-heritage field has a clear sense of who the ideal operator of the camera system is: the ideal photographer has extensive expertise in photographing cultural heritage and in-depth knowledge in information technology and art history.” The research states however that this idea is erroneous as a professional photographer gets quickly bored by the task, and it concludes: “so, if only one person can be committed to this process, it is an art handler”. (Berns, Frey 2005, p. 114). Therefore, museum expert Linda Serenson Colet (2000) recommends in her “Planning an Imaging Project” (part of the series on Quality in Visual Resource Imaging): “Staff the studio with equipment operators who respect the original material but can also work on repetitive production-oriented tasks.”

 

3.2 The relation between the sensor sizes of a still-cameras and the focal length

 

The sensor, situated in the centre of its body, is the heart of a digital camera and its raison d’être. The potential of a sensor depends on the quantity and quality of the photo-diodes. Its physical size limits the range of what can be done with the produced data.

 

The reference point for most reflections on photography is the so-called “full-frame” sensor. The expression refers to the traditional 36 x 24mm frame of a 35mm negative or diapositive film. Sensors which do not have this size are considered “cropped”. They have a so-called “crop factor” (also called “format factor”, “focal length multiplier”, see https://en.wikipedia.org/wiki/Crop_factor).  The “crop factor” indicates how much smaller the sensor’s surface is compared to a full-frame sensor with its 24mm in height and 36mm in length, thus 8,64cm². As most sensors are smaller, their effective “capturing factor” is a fraction of the full-frame size. According to Striewisch (2009, p. 86) many DSLR cameras have a crop factor of 1,5 (the full-frame sensor is 1,5 times bigger than theirs); alternatives are sold by Canon (1,6 and 1,3), Sigma (1,8) and Olympus (2,0).

 

Full-frame sensors are much pricier than “cropped” ones. Due to their size and the often bigger pixels they capture more light in (semi-)dark surroundings (thus less noise), have a wider dynamic range and can thus cover stronger contrasts and preserve details in highlight and shadow sections. However, to benefit from these advantages the camera has to work with more sophisticated, heavier and rather expensive lens-systems. The big sensor invites shooting in manual mode to explore its creative potential. Transfer data to the memory card can take a bit longer (than with other camera types) as more information can be captured which also results in bigger files; however this concerns mainly the rapid shooting of several photos in a row which is rarely the case in slide reproduction. When it comes to producing large pictures for exhibitions the 24 x 36mm surface is a must.

 

Cropped (frame) sensors such as APS-C (Advanced Photo System Type C) or Four-Thirds (the name refers to the obsolete video-camera tube which had a length of 4/3 inch, its imaging area was equal to those of a “4/3”-sensor in a digital camera)   https://en.wikipedia.org/wiki/Four_Thirds_system  benefit from their smaller size when using the same lens as a camera with a full-frame one. A photographic rule says: the “sensor size (or film size) changes the equivalent focal length – not the true focal length of the lens”.   (https://www.bhphotovideo.com/explora/photography/tips-and-solutions/understanding-crop-factor) According to photo-expert Chris Gatcum (2014, p. 183) the “equivalent” or “effective” focal length indicates the field viewing angle of an objective with a lens-system conceived for the large photo-active zone of a “35mm film stock” camera in case it is combined with a smaller sensor. An example: a so-called “normal” objective (fix focal length: 50mm) made for a full-frame sensor (diagonal: 43mm) is used with a cropped one (e.g. Four-Thirds, diagonal: 21,6mm). When the distance between the centre of the lens-system and the sensitive plane (where the image is formed) is kept, but the sensor’s size is reduced, the field viewing angle is also reduced. The objective does not change its “true focal length” (50mm), but adapts optically to the changed circumstances. With its new “effective” focal length (e.g. 80mm which is the result of the focal length 50mm multiplied by the “crop factor” 1,6) a “normal” objective works with the “cropped” sensor as if it was a “tele”. As the “crop factor” gives the number by which the focal length of a lens-system (built for a 36 x 24mm sensor) has to be multiplied to know its “equivalent” focal length when working with a different sensor, it is also called “focal length multiplier”.

 

The interrelation of sensor size and focal length reduces the interchangeability of objectives: cameras with a cropped sensor need other focal lengths to reproduce the same field viewing angle as full-frame ones. The smaller the sensor, the smaller the achievable angle of view and the longer the needed focal length (Hedgecoe 2004, p. 38). The sensor’s size determines which objective is most suitable, with which focal length it could technically achieve the best results. The decision to work with a wide-angle lens could lead to choosing a camera with a less costly cropped sensor, but if one of the target of the slide’s reproduction is to print huge posters, then it is worth considering buying a device with a full-frame sensor.

 

The following overview shows the dimension of different sensor sizes (based on Wunderer 2015, p. 17 and Gatcum, 2014, p. 102, 106). If the sensor is the heart of the camera, the objective is its eye. Their good cooperation guarantees a successful digitisation, their “dissonance” can spoil the quality of the photographic result. Thus both have to be chosen carefully.

 

Name of sensor Sensor format (image area) + aspect ratio Sensor diagonal Sensor surface +

crop factor

Full-frame 36 x 24mm

aspect ratio: 4:3

43,2mm, ideal for 50mm focal length (FL) 8,64cm² = 100%
(Micro) Four-Thirds

 

17,3 x 13mm (for Panasonic, Olympus)

aspect ratio: 4:3

21,6mm, ideal for 25mm FL 2,25cm² = 26% of the full-frame-area

CF = 2,0

Foveon 20,7 x 13,8mm (for Sigma)

aspect ratio: 3:2

24,9mm 2,86cm² = 33%

CF = 1,7

APS-C

(Advanced Photo System Type C)

22,2 x 14,8mm (for Canon)

aspect ratio: 3:2

27mm, ideal for 35mm FL 3,29cm² = 39%

CF = 1,6

APS-C/DX

(Nikon added DX, as its full-frame sensor is called FX (King 2017, p. 37))

23,6 x 15,7mm (for Nikon, Pentax, Fuji, Sony)

aspect ratio: 3:2

28mm, ideal for 35mm FL 3,7cm² = 43%

CF = 1,5

Most compact system cameras 6,2 x 4,6mm 0,29cm² = 3,4%

CF = 5,6

Medium format cameras 48 x 36mm

aspect ratio: 4:3

17,28cm² = 200%

 

The larger the sensor, the better the image quality which can normally be achieved. As a rule one can say: a large sensor produces less noise as more photons can hit the surface; also a bigger surface can leave more space for each pixels, and bigger photo-diodes capture more light. Larger sensors are preferred by professional photographers as they cooperate better with lenses made for DSLR cameras which are still considered “high class”. On the other hand, with a larger sensor used with the same lenses comes a shallower depth of field which reduces the potential of the camera to depict sharply the whole space around the object. With thin slides this is hardly a problem, but with voluminous objects such as optical lantern it can lead to blurring whole sections in its reproduction. A wide-angle lens (e.g. 28mm) used with a APS-C (by 1,5 = 44,8) or a Four-Thirds (by 2,0 = 56) becomes a “normal” objective which changes completely its usability, or the distance between camera and slide has to be adapted which may not be possible with the bought camera-stand. It also seems that small sensors provoke more light diffraction at the edges than bigger ones.

 

The consequences of all these points: it is important to decide before buying / using a digital still-camera what should be done with the created files: to be kept for a long time (which makes migration necessary) or just for a short while (1-2 software generations), to be used for high quality reproduction (e.g. projection, printing) or just for the low-resolution online-database of the archive to inform third parties about the collection. Consequently, the archive has to make sure that the still-camera can / will deliver what is needed. There is a huge range of camera types and models on the market, different in price, manageability and userfriendliness, with a high variety of creative options, produced for all kinds of situations. Informed first by special journals and the internet about test results, it is then interesting to talk to archive colleagues, friends and neighbours and hear more about their experience and recommendation before entering a professional shop for photographic equipment and hold potential candidates in the hands.