Patent GB-711582-A (GB)
Title: Improvements in or relating to photographic enlargers
IPC: G03B27/36
Filed: 1952-06-03
Abstract: 711,582. Photographic enlargers. KODAK, Ltd. June 3, 1952 [June 1, 1951], No. 14040/52. Class 98(1) In a photographic enlarger, a projector lens and a reflector for reflecting light rays through a bent optical path to provide an optical axis having angularly arranged branches, object and image receiving members positioned on different branches and located at conjugate points of the lens when the enlarger is in focus, the lens and one of the members being movable along separate paths and relative to the other member to vary the magnification of the image, are combined with first and second electrical contacts fixed to the image receiving and object members, a third contact fixed with respect to the lens and positioned between the other contacts so as to be in alignment therewith when the enlarger is in focus and said members are at conjugate points, an electric motor, means for connecting the motor to the movable member to move it along one branch relative to the lens, means for connecting the contacts into the motor circuit, a straight bar pivoted to one of the contacts and movable into engagement with the other two contacts when the contacts are aligned and the enlarger is in focus, the construction being such that when the contacts are out of alignment the bar will engage only one of the other contacts to adjust the motor circuit which thereupon moves the movable member relatively to the lens to a conjugate point thereof to bring the enlarger into focus. It is shown that if light from an object at B, Fig. 1, is reflected through 90 degrees at C to form an image at A by means of a lens also at C, A and B are collinear with a point F lying at a distance from each part BC, CA of the light path equal to the focal length f of the lens. When the separation between the lens and the reflector, or the thickness of the lens, are not negligible, or when the angle BCA is not a right-angle, these matters can be allowed for. In Fig. 4, the points B,.A correspond to the centre of the negative 11 and the centre of the easel 12. A bar 13 is pivoted on an earthed pin 16 at B, and touches a contact 14 on the easel at A. When the enlarger is in focus, bar 13 touches a fixed contact 15 at F. A squirrel cage motor has windings 23, 24 with a common connection to A.C. mains at 25. The other ends of the windings are connected together by a resistance 29 and condenser 30, and respectively to contacts 15, 14. When bar 13 is in contact with contacts 14, 15, both windings are earthed and the current passing through them is in phase. When the enlarger is out of focus, bar 13 touches only one of the contacts 14, 15, so that one winding is earthed through condenser 30 and resistance 29, setting up a phase difference so that the motor turns, driving the negative carrier through gearing 19, 20 and a screw 21 so that the enlarger is brought into focus. To allow for the use of lenses of different focal lengths, contact 15 may be adjustable on a slotted guide 42 along the bisector of angle BCA. A scale 51 may be mounted on the easel 12 to indicate the magnification, which depends only on the inclination of rod 13. The lens and mirror C may be fixed, while easel 12 is moved up and down, or the easel may be fixed. The negative carrier may be moved on guides movable vertically with the lens. In a modification, the negative carrier is movable on a base carrying the lamphouse and the lens, which is movable in a direction inclined to the vertical so that the lens is vertically above the centre of the easel only at high magnifications. The guide 42 is also mounted on the base and is so inclined that contact 15 always lies on the bisector of the angle between the two branches of the optical path.