An original picture-taking system – Les Globes de Mercator de l'UNIL

Photographing a spherical surface systematically and accurately is no easy task. The challenges involved include controlling luminosity (chiefly glare) and standardising image capture (register).

To overcome these hurdles, Eric Sapin and Quentin Milliet, both specialists at the School of Forensic Science (ESC), came up with an original system. The spheres’ rotation was controlled by two cross lasers equipped with levels. The beam of one laser passed through the camera’s viewfinder to centre each image perfectly. The other beam was projected to the side to control the globes’ horizontality and verticality permanently.

The pictures followed the latitudes and longitudes starting from Mercator’s prime meridian, which passes through Fuerteventura in the Canary Islands (14.1° W on today’s globes). The 42 final images, centred on selected intersections in the grid, covered each globe entirely.

Positioning the spheres

Each image was captured in relation to the horizon corresponding to a given meridian thanks to the sighting and horizontality lasers, which made it possible to check that the picture remained stable. The first register aligned the globe’s horizon with the real horizon indicated by the laser beam. The second beam, passing through the camera’s viewfinder, showed the optical axis with the latitude and longitude intersection placed at the centre of the image.

Technical parameters

Mamiya Phase One P65+ camera
Macro MF 120 mm 1.4 lens
Aperture F/22
Size of sensor 40.4 x 53.9 mm
8984 x 6732 active pixels

Lighting

To solve the glare problem inherent in photographing a sphere, special lighting was used to yield images with uniform luminosity, colour and sharpness. Four lamps were positioned at 45° angles equidistantly from the centre of the image indicated by the laser beam passing through the camera’s viewfinder.

The lamps were shone on the centre in alternating flashes, first from angles 1 and then from angles 2. The two series of 84 images were then paired, processed to even the luminosity and colours and assembled using Photoshop to create images with no flash glare. Each final image was assembled from a pair by combining the four quadrants with no direct glare around a central point. Next, the images were numbered to facilitate their arrangement and a viewer’s recognition of the regions they covered. Altogether, 423 useful images were obtained for each globe and 84 final high-resolution images would be used for georeferencing.

Depth of field and focusing

For each image, the area photographed was 20 x 20 cm, including a useful central section roughly 16 cm2 to obtain optimal sharpness, uniform luminosity and acceptable distortions due to the curvature of the spheres. The camera was focused one-third behind the first sharp plane and two-thirds in front of the last sharp plane in order to set the field depth on the area captured. The first sharp plane was at the centre of the globes and the last sharp plane on the periphery.

See methods and parameters

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Procédure pour la photographie des globes de Mercator : Procedure for photographing the Mercator globes, Alexandre Hirzel, IT Centre, UNIL | University of Lausanne.