A Reseau plate is a transparent plate with fiduciary markers placed at the focal plane of a camera just in front of the film to provide a means of correcting images to enable them to be used for precision measurement. The crosshairs visible in photos taken on the Moon are an example of this usage.
Potential distortions include the incorrect position of the film in the camera, the distortions of the film by the development process, and (if digitally scanned) the nonlinearity of the image scanner. These errors may be visually imperceptible but limit the precisions of measurements. Because the position of the marks is very precisely known, the image can easily be corrected to account for these effects. When used for photogrammetry, multiple images can be combined using the fiducial marks and knowledge of the camera geometry to enable measurement of distances between objects in the images.
Use on lunar cameras
The Hasselblad Lunar Surface Data Camera was fitted with a Reseau plate. The Reseau plate was made of glass and was fitted to the back of the camera body, extremely close to the film plane. The plate was 5.4 × 5.4 cm in the film plane, which was the useful exposure area on the 70 mm film.
The Reseau plate was engraved with a 5 × 5 grid of crosses. The intersections of the crosses were 10 mm apart and accurately calibrated to a tolerance of 0.002 mm. Except for the double-sized central cross, each of the four arms on a cross was 1 mm long and 0.02 mm wide. The crosses (also known as fiducials) were recorded on every exposed frame and provided a means of determining angular distances between objects in the field-of-view.
When the Hasselblad Lunar Surface Data Camera was fitted with a 60 mm lens, the images of the reseau crosses on the film have an apparent separation of 10.3 degrees. With a 500 mm lens fitted, the apparent separation is 1.24 degrees.