Method for automatically selecting a lens

Abstract

Some of the conventional photographic printers are provided with plurality of lenses with different magifications. In order to change the size of enlargement (or magnification of enlargement), it is necessary for an operator to manually replace a lens or to select a suitable lens among the plural lenses on a revolving lens turret by pressing a selecting button. In this manipulation, the selection of lens and the correction of exposure for the selected lens are left to the operator's judgement. Replacement of lenses in accordance with the change in the size of enlargement involves much trouble in itself. The conventional printers are thus defective in that the printing process does not proceed smoothly. The present invention is to provide an automatic lens selection method for selecting a lens unit among a plurality of lens units having different focal lengths in accordance with a designated magnification of enlargement, which is characterized by the steps of selecting a lens unit having the greatest focal length among the plural lens units, calculating the conjugate length thereof based on the magnification of enlargement and the focal length of the lens, judging whether the calculated conjugate length falls within the effective range of a system, searching a lens unit having the second greatest focal length if the conjugate length falls outside the effective range and repeating the above procedure until a lens unit falling within the effective range is selected.