Rephotography is the act of repeat photography of the same site, with a time lag between the two images; a "then and now" view of a particular area. Some are casual, usually taken from the same view point but without regard to season, lens coverage or framing. Some are very precise and involve a careful study of the original image. The founding work in this style was the Rephotographic Survey project, conceived in 1977 by the project's chief photographer, Mark Klett. This project engaged 120 sites of government survey photographs from the American west first recorded in the 1870's. The resulting book, Second View, The Rephotographic Survey Project, included precise rephotographs of the same locations 100 years later along with an essay by Klett on the methodology and problems encountered with rephotography. Klett revisited these sites a third time for his 2005 book Third View with a new team of photographers including Byron Wolfe, Michael Marshall and Toshi Ueshina.
The accurate rephotographer needs to determine several facts before taking a new image. An important starting point is the choice of the older image. It's usually a good idea to show continuity between the two images by including in the frame a building or other object which is still there in the modern view. Some urban scenes change so much that the original buildings shown have been completely obscured by subsequent skyscrapers, or have been demolished. A "then and now" photograph could be taken but there would be nothing 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 cameras have lenses that 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. Older lenses were softer than their modern equivalents, and usually of a larger aperture, reducing the "wide-angle" feel that modern lenses record.
Through scrutiny of the original image, the rephotographer needs to determine the season and the time of day from observation of the vegetation and the shadows shown in the original view. 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. A good example of this type of rephotography can be seen in the McCord Museum of Canadian History's virtual exhibition "Urban Life through Two Lenses." Another is Douglas Levere's project, "New York Changing", has recently been published. Here Levere rephotograped 114 of Berencie Abbott's, "Changing New York" images.
Rephotography is often used by the scientific world to record the effects of erosion over time, or to measure the extent of sand banks in a river, or other time-related phenomena.