PC Lenses (Perspective Correction) in Digital Photography

We have seen that the place of a scene that is oblique regarding the film appears in the image with escape lines which, in photography, are accented in regard to what we see in plain view, We saw an example in the photograph of the tall building from the street: when pointing the camera upwards to frame, the vertical lines tend to converge in the image towards an escape point as consequence of the plane of the façade of the building and of the film aren't parallel in the moment of the shot. If the façade actually is rectangular, in the photograph it appears as a trapeze shape: the building appears to fall backwards, the angles we know are straight appear deformed, etc.

Lets imagine now that we maintain a parallel between the film and the façade: in the viewfinder only the lower part of the building and street is framed. Nonetheless, we know that the lens produces a circular image of greater coverage than the format, which is why we can suppose that the formed image also includes part of the top of the building although it doesn't appear in the shot. If we achieve, without altering the parallels, find the desired frame in the circular image, eliminating the part we‘re not interested in and include the part we are missing, we‘d obtain an image with parallel vertical lines and he desired frame. Instead of selecting with the frame the central part of the circular image, we'd have to move the film to the desired position. This is precisely what is obtained with PC lenses, where the lens ban be moved parallel to the film to look for the desired frame without inclining the camera, as it happens in large format cameras.