in common to link the two images. The vantage point from which the original photographer took the view may have disappeared over the years, so the rephotographer has to choose an original view for which the vantage point is still accessible, or arrange to rent equipment to duplicate the original position of the camera. Since modern camera lenses differ considerably from older lenses, the rephotographer also has to take into account the area that the lens covers, and the depth of field available. Through scrutiny of the original image, the rephotographer determines the season and the time of day from observation of the vegetation and the shadows shown. The best way to do this is to set up a camera at the original viewpoint, at approximately the right season and time, and wait with the original view in hand, until the shadows reach the same positions relative to surrounding objects. If done with extreme accuracy it should be possible to place one image over the other, and see the edges of buildings match exactly. This type of rephotography can be seen in the McCord Museum of Canadian History's virtual exhibition "Urban Life through Two Lenses". It shows the nineteenth-century views of Montreal by William Notman, rephotographed by Andrzej Maciejewski in 2002. Another is Douglas Levere's project New York Changing; here Levere rephotographed 114 of Berenice Abbott's Changing New York images. Mobile and computationally assisted digital rephotography Smart phones, because they include both cameras and computing, have the potential to simplify the re-photography process. To date, examples of this include "computational rephotography", an approach that uses feature-matching and structure-from-motion to tell a photographer how to move so that the current view best matches the previous view, and "Collaborative Rephotography", which overlays the current view transparently over the original image to allow the photographer to line up elements of the scene before taking the picture. Specific projects include: re.photos, a webportal to create, browse, search, locate, rate, share, and discuss rephotographs; and Retake Melbourne, a crowd-participatory, crowd-funded, mobile app-enabled Deakin University project to rephotograph the fifty-year-old archive of Melbourne (Australia) streetscapes by Mark Strizic. See also Landscape history Time-lapse photography References External links 1850s introductions Photographic techniques Photogrammetry Time