ABSTRACT
Projection printing using an enlarger is facilitated by a true autofocus system, especially if frequent changes of magnification are required for precise composition, or when printing to a specific magnification. In order to reduce the number of separate adjustments involved, autofocus is essential with rectifiers and rectifying enlargers, which incorporate rotational movements of negative and paper planes. An obvious means of autofocus in cameras is by direct measurement or ‘ranging’ of the subject distance by triangulation. The autofocus system may have an ‘infinity lock’ provided to take pictures of subjects through glass, otherwise the lens will focus on the glass. The various systems described normally use a servomotor to operate a focusing control either for unit focusing or front cell focusing. A focus maintenance mode retains sharp focus once this has been set visually and is commonly used for slide projectors and systems such as long focus aerial camera lenses.
