The photosensitive surface should be placed one focal length behind the lens to form images of very distant objects.
Work Step by Step
For very distant objects, the lights rays are coming in almost parallel. As shown in Figure 28.43a, for objects located at "infinity", light rays focus at the focal point of the lens. For objects that are closer, the photosensitive surface should be placed farther from the lens. The same conclusions are drawn from the thin lens equation, discussed in the footnote at the bottom of page 535.