GB902909A - Improvements in or relating to a process camera with automatic electrically controlled accurate focussing - Google Patents

Abstract

902,909. Photographic copying cameras. HOH & HAHNE HOHLUX G.m.b.H. Nov. 4, 1958 [Nov. 4, 1957], No. 35363/58. Class 98 (1). In a process camera wherein the camera back determining the image plane and the copy-holder determining the object plane are movable along the optical axis relatively to the objective, automatic focusing is effected by means of a bridge circuit including two impedances variable in magnitude in accordance with the movements of the camera back and copy-holder, respectively, the output of the unbalanced bridge controlling a drive for one of these two members to restore the balance of the bridge. As shown, a camera with a L-shaped light path has a resistor Ra operated by movement of the copyholder 8 in parallel with a resistor Rb operated by movement of the camera back 9 in one arm of a bridge of which the ratio arms are arranged to be equal, being provided by resistors B 1 , B 2 with a variable balancing resistor B, the fourth arm being formed by a resistor of magnitude appropriate to the focal length of the objective. The amplified out-of-balance output of the bridge detector arm drives a reversible motor 6 for moving the copy-holder 8 after the camera back has been set to a position corresponding to a chosen enlargement ratio, thus satisfying the lens equation 1/a+1/b=1/f. To accommodate the use of objectives of different focal lengths, the appropriate resistor such as R f1 or R f2 is placed in circuit in the fourth bridge arm under control of a switch 13 which may be mechanically or electrically coupled to the objective changing mechanism. Fixed resistors Va, Vb together with resistors Ea, Eb for adjusting the total resistance of the a and b parallel branches to suit the mechanical requirements are provided together with a variable resistor S in the copy-holder branch a to take account of the actual object plane occupied by the original on the copy-holder and a fixed resistor G in the branch a which may be shorted out by a switch 11 closed by movement of the copy-holder to a position closer to the objective than is normal for producing extreme enlargements. Trimmer resistors Ta, Tb, Tf, TG may be provided, resistors Ta, Tb being alternately brought into operation by switches 12 simultaneously with change of objective, switches 12 and 13 being operated together. In a modification, which is not described in detail, two non-adjacent arms of the bridge are made proportional to a - f and b - f while the other two arms are each made proportional to f, so satisfying the alternative lens equation (a-f) x (b-f) = f<SP>2</SP>.