GB2164764A - Motorised zoom lens hood - Google Patents

Abstract

A lens hood (1) includes bellows (9) supported on an expandable folding frame (11). The degree of expansion of the frame in controlled, through a servo motor system (51), responsive to the position of the zoom ring of a zoom lens detected by means of potentiometers (62 and 35), so that the degree of expansion of the lens hood (1) is dependent on a degree of magnification of the zoom lens. <IMAGE>

Description

SPECIFICATION Lens Hood This invention relates to a lens hood, otherwise known as a matte box, lens shade or ray shield.

According to a first aspect of the invention there is provided a lens hood for shading a lens of an image forming apparatus, the hood including means for adjusting the degree of shading of the lens in response to a state of the apparatus.

According to a second aspect of the invention there is provided a matte box including an expandable shading assembly and servo motor means for controlling the position of the shading assembly in response to a demand signal.

Preferably, the lens hood is adapted for use with a zoom lens of a television, of 16mm, camera, the adjusting means being responsive, through a servo motor, system to the position of a zoom ring so that as the lens zooms from a wide to narrow angle of view, or vice versa, the degree of shading provided by the hood is adjusted accordingly.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which; Figure 1 is a side view of an embodiment of the invention Figure 2 is a circuit diagram showing the control circuit for the embodiment of Fig. 1.

Referring to Fig. 1, a lens hood, otherwise known as a matte box, lens shade or ray shield is shown generally designated 1.

The lens hood 1 includes a backplate 3 for attachment to a zoom lens of a camera (not shown). The backplate 3 is preferably adapted to accept the mounting lugs and catch of a standard "Arriflex" filter holder. The backplate 3 also supports a motor gearbox assembly 5 and an expandable lens shading assembly 7.

The assembly 7 includes bellows 9 which are of substantially rectangular cross section supported by an expandable folding frame 11 of "lazy-tongs" construction. The frame 11 comprises two sub-frames 11' prime disposed on opposed sides of the assembly 7, the subassemblies 11' being interconnected.

Each sub assembly 11' includes six support arms 13, 15, 17, 19, 21, 23, which are interconnected through ball race bearings 24, 25, 26. Bearings 24 are connected to backplate 3 thus providing support for the frame 11.

The first arm 13 of the respective subframes 11' are rigidly coupled through a cross yoke 27. The shown first arm 13 includes a part-gear 29 which meshes with a motor pinion 31 connected to a moving coil 1 motor 32 and a position pinion 33 connected to a potentionmeter 35.

Cross tubes between the two sub-frames 11' are provided at points 35, 37.

Arms 21, 23 are connected, through to bearings 26, to a front frame 39. The bearings 24, 26 which are laterally opposed to the motor assembly 5 are mounted in slide bearings 41, 43, so as to allow for lateral expansion of the frame 11.

The bellows 9 are made up from a cloth and plastic formula. They are of tapered construction, the larger end being disposed furthest from the backplate 3. The large and small ends are connected to the front frame 39 and backplate 3 respectively by adhesive.

The backplate 3 and front frame 39 are constructed from machined aluminium alloy finished with anodised black. The arms of the frame assembly 11 are formed from stainless steel.

In use, rotation of the motor 32 causes the lens hood assembly 7 to, expand or contract, by rotating the pinion 31 which in turn states the part gear 29 thus expanding or contracting the frame 11.

The construction is such that the front frame 39 always moves under the control of the folding frame and always remains normal to and on the axis of the hood 1.

The motor/gearbox assembly 5 also includes the electronics and associated controls which are described below with reference to Fig. 2.

In Fig. 2 a servo motor system 51 is shown in which the motor 32 is connected across a pair of differential power operational amplifiers 50, 52. The amplifiers 50, 52 are connected in a bridged arrangement to give push-pull drive to the motor 32.

A non-inverting input of amplifier 50 is connected to a three-position switch 54. When the switch is in position 56, the drive mechanism is off. When the switch is position 58, the motor drive circuit 32. 50. 52 operates to fold up the hood 1. When the switch is in position 60, the motor drive circuit operates to control the lens hood in accordance with the position of a zoom ring on the camera lens (not shown).

Two potentiometers, 62, 35 are provided.

Position potentiometer 35 is responsive to the position of the part gear 29 and thus responds to the position of the front frame 39 of the lens hood 1. Demand potentiometer 62 is connected to the zoom ring of the lens (not shown) by a nylon wheel and neoprene tyre so that, poteniometer 62 follows the rotation of the zoom ring as the lens zooms from a wide to a narrow angle and vice versa. To achieve compatability between lens types, the demand unit may be made adjustable. The tyre is preferably held in contact with the zoom ring by spring retention. The torque required to turn the potentiometer 62 is extremely low and therefore absorbs neglible power from the lens system. The potentiometer 62 is further connected between variable resistance rotary controls 64, 66 which set the upper and lower limits of the hood extension.

Terminal 58 is connected to the source voltage via a further resistance 68.

In use the switch is set from the off position 56 to the operational position 60. The operation amplifier 50 will then be responsive to the voltage difference between potentiometers 62, 35 causing the motor to turn in one direction or the other, depending upon the voltage difference, thus adjusting the position of the bellows unit 7, through gears 31, 29. Movement of the bellows unit is sensed by piotentiometer 35, through pinion 33 the electrical resistance of which is thus adjusted until the voltage difference between the two potentometers 62, 35 is reduced to zero, when the amplifier 50, 52 stops driving the motor.

Different positions of the zoom ring will result in the potentiometer 62 having different values of electrical resistance, which will therefore resuit in the lens hood 1 having different degrees of extension, changes in the position of the zoom ring resulting in corresponding changes in the lens hood extension through the servo control circuit provided by the motor 32, op amps 50, 52 and position potentiometer 35.

If it is desired to fold the lens hood 1 to its minimum extension, the switch is turned to position 58 and a constant non-controlled voltage, is applied to op amp 50 which causes the motor 32 to turn and retract the assembly 11 and thus fold the lens hood 1 for storage.

The rotary control 64, 66 and three position switch 54 are preferably mounted on the top of the unit 5.

Power of the circuit is preferably 12 volts d.c. provided by a separate nickel-cadmium pack, or mains power supply. Provision may also be made for operating the lens hood 1 from camera power when this is available.

The circuit is preferably constructed as a printed circuit board.

Claims (8)

1. A lens hood attachable to image forming apparatus for shading a lens thereof, the hood comprising an expandable shading assembly and control means, including a motor, for controlling the degree of expansion of the shading assembly in response to a control signal, indicative of a desired shading requirement.

2. A lens hood as claimed in claim 1 wherein the hood is arranged to be attached to a zoom lens of the image forming apparatus and the control signal is generated in response to the magnification produced by the zoom lens.

3. A lens hood as claimed in claim 1 or claim 2 wherein the control means includes differential amplifying means connected to the motor and responsive to both a first potentiometer for generating said control signal and a second potentiometer for generating a feedback signal responsive to the degree of expansion of the shading assembly with the control means being arranged so that the motor adjusts the degree of expansion of the shading assembly as sensed by the second potentiometer until the degree of expansion of the assembly is in accordance with the degree of expansion indicated by the control signal of the first potentiometer.

4. A lens hood as claimed in any one of the preceding claims wherein the control means further includes selectively engageable means for contracting the shading assembly to a minimum degree of expansion irrespective of the state of the control signal.

5. A lens hood as claimed in any one of the preceding claims wherein the control means further includes means for setting the upper and lower limits of expansion of the shading assembly.

6. A lens hood as claimed in claim 5 dependent directly or indirectly on claim 3 wherein the setting means comprises first and second variable resistors, disposed on either side of the second potentiometer in a ladder arrangement.

7. A lens hood as claimed in any one of the preceding claims wherein the shading assembly includes bellows supported by an expandable folding frame, the frame being connected to the motor via a gearing assembly so that the degree of expansion of the frame is determined by the operation of the motor.

8. A lens hood substantially as hereinbefore described with reference to the accompanying drawings.