Available online 22 November 2013
Macro Photography for Reflectance Transformation Imaging:
A Practical Guide to the Highlights Method
Macro photography is extreme close-up photography in which the size of the subject on the image sensor is greater than life size. This challenging photography requires ad hoc hardware. This section aims to introduce the specific technical requirements to obtain macro photographs of areas of about 1 cm2 that are suitable for the RTI Highlights method.
In order to shoot RTI images on the order of 1 cm2, a reference sphere is required to allow the RTI software to determine the direction of the incident light. Its dimension should be small enough, a diameter of less than 1 mm, in order not to cast shadow on the area of interest and to be on focus within the area itself. Indeed, macro photography has an extremely narrow depth of field. Ball bearings are usually a good option although their smallest size is of only 1 mm. Ball point pens labeled as “fine” provide spheres smaller than 1 mm that are well suited for this technique (Figure 1). Indeed, there are even ballpoint pens of less than 0,5 mm. These are called micro tip pens, and currently the smallest is the Pilot G-Tec-C with 0,25 mm. However, the handling and extraction of these small ballpoints is a daunting task and, therefore, it is not recommended.
Macro photography requires intense lighting. It is also necessary to shoot with fast exposure to avoid registering the movements of the handheld light source or any environmental vibrations. Flash lights are therefore the best choice, particularly in case of light-sensitive works of art. These could be either speedlights or studio strobes which both can be triggered remotely from the camera (Figure 2). RTI requires about 30 shots for each scene, so the faster recycle time between each shot and the lack of batteries are two reasons to favor the studio strobes against speedlights in order to make the overall RTI shooting process much faster. Furthermore, the speedlights can be damaged without sufficient cooling time, so one would need to avoid shooting too fast with these. There are lightweight and small studio strobes that fit the macro photography RTI lighting requirements. Studio strobes also feature a continuous pilot light which is useful and necessary for focusing. When using speedlights, a LED spotlight on a gooseneck is generally sufficient for focusing.
2.3. Photo Editing
When taking macro images it is likely that the camera will move on the order of hundreds of microns during the RTI shooting section. If that happens, images will not be suitable for RTI. Nevertheless, they can be still lined manually or automatically with an image editing software. In Photoshop, for example, the script “load images into stack” and checking “allow automatic alignment” can be used. Photoshop will create a file with the aligned images on levels. Then, the images can be saved automatically: FILE/SCRIPT/ Export layers to file.
USB microscopes are fine for capturing macro images as they can be focused at distances of about 1,5 cm, which is sufficient to allow for the RTI lighting that is needed at different angles . However, due to their low pixel count, they provide images with a resolution that is too small for the RTI Highlights method. For this reason, it is better to use a digital camera with a high pixel count such as the one used in this paper, the Nikon D800 with 36 megapixels. Indeed, with such a big sensor, it is possible to photograph a large area that encompasses both the sphere and the area of interest, even as these two are far away so that the sphere does not cast a shadow on the area of interest. Among the advantages of digital cameras over USB microscopes, is the possibility to do multispectral imaging RTI. As for further developments, most suited would be the new mirrorless cameras since the vibration of the mirror in the DSLR cameras contributes to a slight shaking of the camera.
Before dealing with the section on lenses, some other photographic tools necessary for macro photography  are: the macro rail, a tripod, tethering and the mirror-up shooting mode. Macro rail is recommended for fine focusing since the lens focusing ring may not be sufficient to achieve the desired sharpness and some of the macro photography methods can only focus by moving the camera. The tripod must be as sturdy as possible and must allow to reverse the tube to get a more stable configuration (Figure 3). Although not mandatory, tethering the camera to a computer allows effortless checking of focus and correct lighting with the “liveview” function. Mirror-up shooting mode keeps the mirror up, as it is called in Nikon cameras, for avoiding vibration during the shooting. For Nikon cameras, a remote shutter is required to shoot in this mode when the camera is tethered to a computer.
From left to right:
Figure 1. The image, obtained with a Veho USB microscope, compares the sphere of a papermate grip ball point pen labeled “fine” (left) and a small ball bearing of 1 mm (right).
Figure 2. Left: compact and lightweight studio strobe, 180W flash (guide number 45) and 75W modeling bulb. Right: Speedlight Nikon SB-600. This was used for the RTI documentation of the Ingels collection presented in this paper.
Figure 3. Ingels Collection, Sweden. RTI set up with the camera mounted on macro rail and tripod with reversed tube.
In order to produce test images on a printed document that can be easily acquired for comparison, the lenses have been tested imaging one specific area of a 10 euro bill (Figure 4).
From left to right:
Figure 5. Nikon D800 with some macro set ups. Left to right: reverse ring and 20 mm lens; Otamat Macro Lens; 200 mm lens with Microscope objective adapter and 2X objective.
Figure 6. From left to right: close-up lens, macro extension tube, microscope objective adapter, teleconverter, reverse ring and coupling ring.
Table I. Comparison of macro photography methods for RTI imaging.
2. Macro Extension tube
4. Microscope objective adapter
From left to right:
Figure 8. Macro detail of the 10 euro bill (left) and set up (right) with teleconverter 2X (1), 300 mm lens (2), microscope objective adapter (3) and 10X objective (4).
Figure 9. Up to down, left to right: 200mm lens/f8, the sphere is out of focus; 200mm lens/f16, sphere and paper are in focus but vignetting is showing; 300mm lens/f4.5, sphere and paper are in focus without vignetting; 300mm lens/f5.6, sphere and paper are in focus but vignetting is showing. Far right: set up with 200 mm lens, microscope objective adapter and 2X objective.
5. Coupling ring
A reverse ring is plugged onto a wide angle lens. With a 20 mm lens, the working distance is about 4,5 cm (Figure 11). Even if the magnification is not as much as with other tools, this method is preferred since it is lightweight which means less vibration. Its simplicity also delivers very sharp images.
Left to right, up to down:
Figure 11. Macro detail of the 10 euro bill (left) and set up (right) with 200 mm lens (1) and reverse ring (2).
Figure 12. Macro detail of the 10 euro bill (left) and set up (right) with Otamat 2X lens.
Figure 13. Chinese coins of northern Sung Dynasty 960-1127 A.D, photo taken with a 49 mm macro extension tube and 200 mm lens, working distance 1.2 m, black reference sphere diameter 15mm.
1493, Nuremberg Chronicle
From left to right:
Figure 14. Nuremberg Chronicle, dated 1493. Macro RTI enhances the fibrous appearance of the rags paper.
Figure 15. Book of Hours, dated 1498. Macro photo, edge rims characteristic of woodcuts relief printing are evident such as the lack of textile fibers in the parchment.
Figure 16. Arch of Triumph, 1517. Macro photo. Even if made with a much more refined technique, the lines show no regularity of width or direction, revealing this print as a woodcut.
1786, Pantheon in Rome
1909, Portrait of C. G. Liljevalch
Figure 17 (left). Pantheon in Rome, 1786. Etching shows its characteristics: raised ink and varying intensity of the lines.
Figure 18 (right). Portrait of C. G. Liljevalch, 1909. Macro RTI photo. Engraving showing its characteristics: raised ink, tapering and swelling lines.
3.4. Asian prints
XIX century blockprints
Figure 19 (left). Macro RTI image of a Japanese blockprint, 1828. The rice paper texture is evident.
Figure 20 (right). Macro RTI image of Three Lamas, Tibetan blockprint, XIX century. The rice paper texture is evident.
This work was only possible thanks to the generous support of the Ingels Collection, Sweden, which made the materials available for the study and funded the research.
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Dr. Antonino Cosentino is a PhD Physicist specialized in Cultural Heritage Science and his goal is to promote innovative and affordable instrumental solutions for technical documentation and examination of Art. He’s currently working on his private practice service providing art documentation, training and consulting for private professionals and institutions and he’s blogging on Cultural Heritage Science Open Source, chsopensource.org.