Patent GB-578393-A (GB)
Title: Improvements relating to photographic enlargers
IPC: G03B27/36
Filed: 1944-05-02
Abstract: 578,393. Photographic enlarging apparatus ; lens fittings. PULLIN OPTICAL CO., Ltd., and BAIGENT, E. W. May 2, 1944, No. 8269. [Class 98 (i)] In an enlarger in which a carriage for the negative and objective holders is adjustable to and from the support for the sensitive sheet, with provision for automatic relative focusing adjustment between the negative and objective holders, this provision comprises a parallelogram linkage having a pair of opposite links long compared with the relative movement required, said links being pivoted at one end to the carriage, and at the other end to the member (negative holder or objective holder.) which is movable for focusing, the linkage being turned about its pivots on the carriage by a cam which is moved in accordance with the move-. ments of the carriage to and from the support for the sensitive sheet. Provision may also be made for permitting frictionless relative floating movement between the negative and objective holders ; for dispensing with a counterweight for the carriage of a vertical enlarger by causing the hand wheel which displaces the carriage up and down to be automatically clutched to the carriage except when it is being operated ; and for putting the automatic focusing mechanism out of action to permit manual focusing of the objective. As shown, a base 1, Fig. 1, with rubber feet 2 supports in a socket 1 a vertical pillar 3 of hexagonal section upon which is guided a carriage 4 displaceable up and down by rack-and-pinion gearing operated by a hand wheel 8. Pivoted to the carriage at 18, 19 are long parallel.links 14, 15 pivotally supporting at their free ends an objective holder 21 floating, without frictional engagement, in a tubular extension 22, Fig. 3, of a negative holder 9 secured to the carriage 4. A snail cam 24 pivoted in the carriage is coupled by a gearwheel train to the rack pinion operated by the hand wheel 8, and-actuates the linkage, as the hand wheel is turned, by reason of a roller 34 on the link 14 resting upon the cam. A clock spring around the pivot of the cam prevents backlash in the gear-wheel train. When non-automatic focusing is desired, the linkage is raised to lift the roller 34 off the cam, and is held in the off position by a spring latch 56, Fig. 3, on the negative holder extension 22 entering-into locking engagement with a keyhole slot in the objective holder 21 ; thereafter the objective may be manually focused by turning a milled ring. 54 which displaces the objective axially in its holder 21 through screw gearing 55, 63. A second spring latch 64 serves to lock the objective in position in its holder 21 when set for automatic focusing. The objective has a diaphragm adjusting ring 52 arrested in its various settings by a ball catch 52a ; and the link 15 supports on a post 67 a colour filter holder 66 which may be swung around the post into or out of operative position before the objective. The negative holder 9 supports a condenser mounting 11 which in turn detachably supports one or other of two lamp houses, of which 10 has one axially-disposed lamp 13, and 10a, has several lamps 13a arranged around the axis to provide more diffuse lighting. The lamp flex connects to a socket on the top of the pillar 3 through which connections pass to the base 1. The carriage may be counterbalanced by a weight attached to a cord passing over a pulley on the pillar, but clutch provision for avoiding the use of a counterweight .may consist in mounting the hand wheel 8, Fig. 2, slidably upon its spindle 7, with a driving feather 46, and providing between the boss of the hand wheel and a boss 38'on the carriage conical engaging surfaces with a brake fabric lining 40. A compression spring 41 disposed between a collar 51 on the spindle 7 and shoulder 42 in the wheel boss urges the wheel into clutching engagement with the boss 38. When the hand wheel is gripped for the purpose of displacing the carriage 4 on its pillar 3, one or more of several plungers 48 radially disposed in the hand wheel 8 are pressed inwardly : the inner ends of the plungers are inclined to engage a conical surface of the collar 51 on the fixed spindle 7, with the result that the inward movement of the plungers causes the hand wheel to move axially on the spindle against the action of the spring 41 so as to disengage the clutch and allow the hand wheel to be turned. On releasing the hand wheel, the spring 41 reacts to force the plungers out again and reverse the axial displacement of the wheel until the clutch members re-engage. The base 1 may have a tiltable paper &c. support for correcting distortions in the negative, and may carry a superimposed frame for use in printing in cloud effects &c. The pillar socket 1a permits the carriage and pillar to be swung about the axis of the pillar to displace the carriage out of vertical alignment with the base 1 when extra large enlargement is required, e.g. on a sensitive sheet placed on the floor.