Rangefinder

Abstract

A rangefinding assembly, which comprises a transmitter having emitting means for emitting short waves. A receiver is mounted in fixed relation with respect to the transmitter, which receiver is adapted to receive short waves emitted by the transmitter and reflected by an object. An image forming system is adapted to project the waves reflected from the object on the receiver. The image forming system defines an axis. The axis, the transmitter and the receiver define a plane adapted to pass through the object. The receiver has at least two receiving zones. At least a part of the receiving zones is disposed in the plane and is at least partly offset from the axis, whereby each receiving zone is assigned to a different distance range of the object and delivers a specific output signal, when admitted by the short waves reflected from the object. Transducing means are arranged within at least a part of the receiving zones and are adapted to produce specific output signals in response to the short waves.

Description

This invention relates to a rangefinder comprising a transmitter for beamed short waves, preferably electromagnetic waves, and a receiver, and preferably comprising two transducers, which convert the energy emitted by the transmitter into an electrical parameter, and, if desired, a first image-forming system, which forms an image of the emitting surface of the transmitter on an object, and a second image-forming system, which forms on the transducers an image of the image formed on the object, whereby in particular the transducers are disposed in the plane, which is defined by the axes of the transmitter and receiver and offset from the axis of the image-forming system of the receiver and whereby further the axis of the beam emitted by the transmitter and the axis of the image-forming system of the receiver intersect, if desired, at a finite distance, and the image formed by the second image-forming system exhibits a parallax, which depends on the object distance.

In a known rangefinder of the kind described hereinbefore the transmitter consists of a light source and an optical system is provided to form an image of the light source on the object. The receiver comprises a second optical system and a sharp edge, which is disposed in the image plane of the second optical system. Two photoconductors are arranged on that side of the sharp edge, which is remote from the optical system and symmetrically disposed with respect to the optical axis of the optical system. When an image of the light spot formed on the object is formed in the plane of the above-mentioned sharp edge, both photoconductors will be uniformly illuminated. When an image of the light spot is formed in front of the sharp edge, one photoconductor will be preferentially illuminated. When the image is formed behind the sharp edge, light will be preferentially received by the other photoconductor. A comparison of the quantities of light, which are incident on the two photoconductors may be used to control a positioning motor, which displaces the optical system of the receiver along the optical axis until equal quantities of light fall on both photoconductors. The sensitivity of that rangefinder depends primarily on the aperture of the optical system of the receiver. To provide a sufficiently exact measurement in a range of, e.g. 1 - 20 meters, it will be necessary to select an optical system, which has a relatively large relative aperture. This will involve a high display, particularly because the aberrations of the optical system should be minimized, and relatively large overall dimensions of the instrument. An arrangement of the kind described above has also already been known, whereby the axis of the beam of the emitter and the axis of the image-forming system for the receiver are so long deviated relatively to each other, until the beam reflected by the object reaches the receiver. The displacement of the focusing member of the lens was coupled with the movement of the emitter and/or the receiver, so that the lens was focused, when the reflected beam reached the receiver. This arrangement involved a relatively high display for the different movable parts. This complicated gear however had also to be adjusted carefully and was rather susceptible to troubles.

According to the invention these difficulties are avoided in that the receiver and the emitter are arranged essentially stationarily with respect to each other, whereby the receiver -- as already known -- shows zones assigned to variable object distances, which, upon equally strong impingement by the beam of the emitter, generate specific output signals, which may be of different value or may be applied to different pilot channels, and whereby preferably the emitter generates short-time pulses, as it is already known.

Because the transducers for visible light and the adjacent spectral ranges may be extremely small, the receiver will be very compact. The sensitivity of the rangefinder will primarily depend on the distance of the receiver from the transmitter. It will not be difficult to provide a sufficiently large base line for the measurement, particularly if the instrument is structurally combined with other devices, such as cameras, binoculars, etc., because these devices are so dimensioned as to afford a sufficiently large base line.

In a desirable embodiment of the invention, the transmitter emits a short-time pulse. Each transducer feeds preferably through an amplifier stage a data storage device and these storage devices are connected to an indicating means and/or a positioning means. The pulsed operation of the transmitter has the advantage that only a relatively small power is required, so that this rangefinder can be used also in portable devices, which are fed by low-capacity batteries. To ensure that the operation does not depend on environmental conditions, it has proved desirable to measure not the absolute value of the energy, which is incident on the transducer, but the energy change, which accompanies the emitted pulse. In a preferred embodiment of the invention, a differentiating stage, such as a coupling capacitor, is provided for this purpose between the transducer and the storage device.

In a preferred embodiment of the invention, the data storage device consists of a bistable switching stage, e.g., a bistable multivibrator, which is shifted from a first state into a second one, when the output signal of the transducer or a signal derived from said output signal exceeds a predetermined threshold value. If the rangefinder operates in the visible range of the spectrum or in the near infrared or ultraviolet ranges, the transmitter may preferably consist of a gas discharge lamp. According to another embodiment, the transmitter consists of a luminescent semiconductor diode, e.g., a gallium arsenide diode. These luminescent diodes have extremely small dimensions and a relatively high efficiency. It will be a special advantage that the light-emitting surface is extremely small, so that the transmitter rays can be very sharply beamed. Besides, the above-mentioned gallium arsenide diode has the advantage that it emits virtually monochromatic radiation having a wave length of about 900 nanometers, which is in the infrared range.

Further features of the invention will become apparent from the following description of several illustrative embodiments and the drawing.

In FIG. 1 a rangefinder for pulsed operation is illustrated,

FIGS. 2, 3 and 4 are block diagrams illustrating the means for utilizing the signals of the receiver in a rangefinder as shown in FIG. 1,

FIG. 5 shows a detail of the circuit of the arrangement shown in FIG. 4,

FIGS. 6 and 7 show also diagrammatically two embodiments of the novel rangefinder as applied to motion picture or photographic cameras,

FIG. 8 illustrates a modification of the arrangement shown in FIG. 7,

FIG. 9 shows diagrammatically another embodiment of the invention.

The rangefinder, which is shown in FIG. 1 gives a measurement in a predetermined sequence. Since it is not required, as a rule, to provide an indication, which is continuous throughout the measuring range, it will be sufficient in general, if the distance is indicated in certain steps. In such cases the operation of the transmitter will be pulsed rather than continuous and the power requirement of the transmitter can be reduced to a fraction of the power, which is required for a continuous operation. FIG. 1 shows diagrammatically such an inventive rangefinder for pulsed operation. The transmitter consists of a gas discharge lamp 29, which has associated with it a power supply unit comprising a capacitor 30 and an associated charging device 31. The ignition switch is indicated at 32. An image of the electric arc produced by the gas discharge lamp is formed in the object space by a lens 33. The receiver comprises also a lens 34 and a number of photodiodes 35, which are disposed in the image plane of the lens. It may be desirable to provide filter discs 36a and 37a before the lenses of the transmitter and receiver, respectively. The photodiodes 35 are connected to a data storage device 36, which controls an indicator 37. The transmitter and receiver are rigidly mounted. To perform a measurement, the key 32 is pressed to cause a discharge of the capacitor 30 through the gas discharge lamp 29, so that a defined, relatively small zone of the object 28 is illuminated. An image of this light spot is formed by the lens 34 of the receiver on the photodiodes 35. It will depend on the position of the object 28, which one of the photodiodes receives the light, which is emitted by the transmitter; the remaining photodiodes are not illuminated by the transmitter. If the object 28 is disposed at the intersection of the optical axes of the transmitter and receiver, light will be incident on the optical axis of the receiver. If the object 28 is more distant, light will be incident on a photodiode, which is above the optical axis. If the object lies before the intersection of the optical axes, the light pulse emitted by the transmitter will be incident on a photodiode, which is below the optical axis of the receiver. Since the object 28 has normally a certain illumination, which is due to the ambient light and causes light to be incident on all photodiodes 35, the parameter, which is measured, is not the absolute value of the quantity of light, which is incident on the photodiodes, but the change of such quantity with time. The change of the output of the photodiodes 35 is amplified and stored in the data storage device 36. The indicator 37 indicates at any time the result of the last rangefinding operation. The switch 32 may be operated by hand. Alternatively, it may be desirable to operate the transmitter with a certain pulse train. The frequency of that pulse train will depend on the relative velocity between the rangefinder and the object 28.

FIG. 2 is a block diagram, which represents the data storage device 36. Each photodiode 35 is connected by a coupling capacitor 38 to a bistable switching device 39, which may consist of a multivibrator or of a Schmitt trigger. Each switching stage is connected to a tap of a series connection of resistors 40. When one of the photodiodes 35 is excited by a light pulse, its output will be transmitted by the coupling capacitor 38 to the associated switching stage 39, so that the same assumes its set state, in which it grounds the associated tap of the series connection of resistors 40. The arrangement is such that the photodiode, which is associated with the largest range of distances, connects the smallest resistance, whereas the switching stage, which is associated with the smallest range of distances, bridges the largest resistance. The effective resistance of the series connection of resistors is thus a measure of the distance from the rangefinder to the object. If the distance from the rangefinder to the object 28 is os large that the reflected light is not sufficient to excite a switching stage, the total resistance of the series connection of resistors 40 will be effective. That resistance could be associated e.g., with an infinite distance. For the actual indication of the distance, a resistance-measuring device might be used, which comprises a measuring instrument 37 carrying a distance scale. FIG. 2 shows an indicating bridge network for a resistance measurement. Any other resistance-measuring circuit may be used, such as a cross-coil instrument and the like.

FIG. 3 shows a modification of the circuit, which is shown in FIG. 2. In this embodiment, incandescent bulbs 41 are switched by the switching stage 39. When a light pulse has been emitted, that incandescent bulb 41 is energized, which is associated with the distance range in which the object 28 is disposed. It will be desirable to provide in the power supply part of the storage stage 36 a switch, which is opened for a short time immediately before a new measuring pulse is transmitted. The opening of that switch causes the canceling of the result of the original measurement. Such switch may consist of a wiping switch and may be coupled to the initiating switch 32.

FIG. 4 is a circuit diagram showing another modification of the embodiment of FIG. 1. A transmitter 42 is connected by a main switch 43 to a power source, not shown. The transmitter comprises an ignition switch 44, which is connected to a switch 45 included in the circuit of the measuring and storage unit. The photodiodes 35 are included in the input circuit of respective amplifiers 46, which just as in the above-described embodiment are connected by respective coupling capacitors 37 to a bistable multivibrator 48 (flip-flop). The multivibrators 48 cooperate with a respective controller 49, which includes in its output circuit a motor 50 and a relay 51. The multivibrators 48 have a preferential state, so that they assume initially a defined position of rest, when the supply voltage is applied. This arrangement has substantially the following mode of operation:

When the ignition switch 44 is operated, the contact 45 is closed first to energize the amplifiers, the multivibrators and the controller. The contact 44 is then closed to initiate the flash. Depending on the position of the object 28, one of the photodiodes 35 is excited to shift the associated multivibrator to its operative position. The resistance value thus defined is compared in the controller 49 with a setting, which corresponds to the position of the motor 50. If that position is in agreement with the measured value, the motor 50 and the relay 51 will remain deenergized, so that the amplifiers, multivibrators and controller are deenergized, when the key 44 is released. The measured value will in most cases fail to correspond to the pregiven position of the motor 50. Depending on the direction of the deviation, the motor will then be energized to operate in one sense of rotation or the other. The motor 50 will be operated until a feedback device indicates that the position of the motor corresponds to the measured value. The relay 51 remains excited as long as the motor is energized. The normally open contact 52 of the relay 51 connects the amplifiers, multivibrators and controller to the source of power regardless of the position of switch 44 and will not interrupt the circuit until the motor 50 has reached the position corresponding to the object distance.

FIG. 5 is a basic circuit diagram of the arrangement just described. The photodiode 35 is connected to the base of a first transistor stage 53, the collector circuit of which is coupled by the capacitor 47 to the bistable multivibrator. The latter comprises two transistors 54, 55. In known manner, the collector circuit of one transistor is connected by a resistor to the base circuit of the other. A diode 56 connects the multivibrator to a tap of the series connection of resistors 40. As in FIG. 2, that series connection is included in a Wheatstone bridge, which contains in its other arms the resistors 57, 58 and 59. The resistors 57 and 58 have fixed values. The resistor 59 is adjustable by the motor 50. A differential amplifier connected across the diagonal of the bridge comprises two transistors 60 and 61. The motor 50 and the relay 51 are included in the output circuit of the transistors 60 and 61. The motor 50 may be connected to an indicating instrument or to a mechanism for focusing a lens.

It will be desirable to form the photodiodes 35, the preamplifier 53, the coupling capacitor 47 and the multivibrator 54, 55 on a common support, or carrier means preferably a semiconductor crystal, by the integrated switching circuit technique. The individual stages may constitute integrated switching circuits, alternatively, all stages may be combined in one such switching circuit, which may include also the differential amplifier 60, 61, if desired.

FIG. 6 illustrates the use of the novel rangefinder in a cinematographic camera. The optical system of the camera consists of a prime lens 62 and an auxiliary lens 63 having a variable magnification. The front lens element 64 of the auxiliary lens 63 is axially movable to focus the optical system. A partially reflecting prism 65 is disposed between the prime lens 62 and the auxiliary lens. Part of the light, which falls through the auxiliary lens is deflected by the prism 65 into a viewfinder 66. A rotating shutter 68 is disposed between the prime lens 62 and an exposure aperture 67 and covers the latter as the film is advanced. A contact roller 70 is carried by the shaft 69 of the rotating shutter 68 and cooperates with stationary contacts 71 to connect the same as the film is advanced and the exposure aperture is covered. The transmitter and receiver are disposed on the opposite side of the optical axis of the lens. The transmitter comprises a lens 72. A luminescent diode 73, particularly a gallium arsenide diode, is disposed in the image plane of the lens 72 and comprises a power supply unit 74. The ignition switch of the transmitter is designated 75. The receiver comprises a lens 76 and photodiodes 77 disposed in the image plane of the lens 76 and controlling a data storage device 78. The photodiodes have a spectral sensivity, which is selected to match the transmitter. Silicon photodiodes are desirable for use with gallium arsenide diodes, because they have a maximum sensitivity in the infrared range. The data storage device controls by means of a Wheatstone bridge 79 a motor 80 in the manner described above. The motor 80 displaces the front lens element 64 of the lens in an axial direction. For a feedback of the existing focus setting, the front lens element is connected to a variable resistance 81 of the bridge 79. Where a luminescent diode 73 is used, which emits light that would result in a response of the film, it will be recommendable to synchronize the light pulses with the operation of the shutter 68 of the camera, so that no light pulse will be emitted, unless the shutter 68 covers the exposure aperture. For this purpose, the switch consisting of the contact roller 70 and the stationary contacts 71 is included in the ignition circuit of the transmitter. Because the object space includes normally a large number of objects at different distances, an area is defined in the viewfinder in known manner, which corresponds to the measuring field of the rangefinder. The ignition switch 75 may be manually operable or may be automatically controlled at a certain pulse frequency, which may be adjustable, if desired. The size of the light-receiving areas of the photodiodes 77 is selected to match the range, in which the depth of field of the camera lens is most critical. This means that the object distance range corresponding to the width of the light-receiving surface must be smaller than the depth of field, when the camera is set to the most critical values of the focal length and stop. The use of luminescent diodes rather than a gas discharge lamp in the transmitter has the advantage that a relatively low feed voltage is sufficient. Besides, the dimensions of that transmitter element are much smaller than those of comparable other light sources, so that the information of an exact image is particularly facilitated. Gallium arsenide diodes emit light, which has only a very small band width, so that it is virtually monochromatic, and this light is in the infrared range so that persons and animals in the object space will not become aware of the rangefinding operation. Reference is also made to the relatively high efficiency of gallium arsenide diodes and to the fact that they are highly suitable for pulsed operation.

In photographic and cinematographic operations it is often not necessary for the operator to know the absolute value of the object distance. Automatic focusing is sometimes inconsistent with an optimum composition of the image. In this case a device may be employed, which indicates to the user of the rangefinder, whether the object is disposed in the zone of depth of field of the lens 82. The focusing device 83, the zooming device 84 and the means 85 for adjusting the diaphragm 86 associated with the lens are coupled to a depth-of-field calculator 87, which may consist of a mechanical calculating mechanism or an electric calculating circuit. The depth-of-field calculator 87 controls a pinion 88, which adjusts two racks 89 and 90 in opposite senses. Each of the two racks 89 and 90 carries a photodiode 91 and 92. The two photodiodes are so adjusted by the depth-of-field calculator 87 that they correspond respectively to the spaces disposed outside of the zone of depth of field. The photodiode 92 is associated with the space before the zone of depth of field and the photodiode 91 with the space which succeeds the zone of depth of field. The diode 91 or 92 will receive a light pulse during the measurement if the object is outside of the zone of depth of field. This deviation is indicated by suitable signalling means.

FIG. 8 shows a modification of the arrangement that has been described hereinbefore. The pinion 88 of the depth-of-field calculator 87 operates two racks 93, 94 to adjust stop blades 95 and 96, which precede a photodiode 97 and a photoconductor, respectively. The stop blades 95 and 96 are so controlled that their edges correspond to the limits of the zone of depth of field. If the object is disposed outside the depth of field, an image of the light spot formed by the transmitter 73 on the object will be formed on the stop blade 95 or 96. A suitable alarm device can be operated, when the receiver fails to receive a signal.

Another desirable embodiment of the invention is shown in FIG. 9. An image of the transmitter 100 is formed by a lens 101 on the object 102 to be measured. The image of the transmitter which is produced on the object is formed by the lens 103 of the receiver. To ensure the formation of sharp images of the particular objects, regardless of their distance, the plane 104 in which the transducers are disposed is inclined relative to the optical axis in accordance with Scheimpflug's conditions. The plane 104 is defined in known manner by the intersection 105 of the main plane of the lens of the receiver and the plane, which contains the several objects of which sharp images are to be formed. That plane thus includes the optical axis of the transmitter.

Where photoconductors are used as photoelectric transducers, a large useful signal will be obtained, if each photoconductor is included in a voltage divider circuit, which comprises a second resistor that consists also of a photoconductor which receives light from the environment of the lens. Where nonlinear photoelectric transducers are used, they will preferably be operated at their optimum operating point. To eliminate the influence of the illumination of the object by the ambient light, the receiver is preferably preceded by a dimmer, which is controlled by a photovoltaic cell, a photoconductor or the like and by which a diaphragm or a neutral wedge preceding the photoelectric transducer is adjusted so that the average of the luminous flux falling on the photoelectric transducers is constant. Such a dimmer may be designed like the automatic diaphragm control device, which is known in photographic and cinematographic cameras. Where the novel rangefinder is used in combination with photographic or cinematographic lenses, it will be desirable to arrange the receiver in the path of the light passing through the lens, or to use reflecting means, whereby the light required for the receiver is deflected out of the path of light passing through the lens. If the receiver of the rangefinder or the beam-splitting mirror succeeds the diaphragm of the lens, the average quantity of the light, which is incident on the photoelectric transducer of the receiver will be constant regardless of the illumination of the object by the ambient light.

The invention is not restricted to the examples, which have been described hereinbefore. In addition to photography, cinematography and television, the invention may be used also for geodetic surveying instruments, as well as for sighting and tracking devices. Besides, the novel device may be used for a measurement of altitude in known instrument landing operations.

Various lasers may be used rather than the transmitters described hereinbefore, ruby lasers and CO.sub.2 lasers being particularly desirable, because they have a favorable frequency range.

Claims

What is claimed is:

1. A rangefinder assembly, comprising a transmitter having emitting means for emitting short electromagnetic waves,

a receiver mounted in fixed relation with respect to said transmitter, said receiver being adapted to receive short electromagnetic waves emitted by said transmitter and reflected by an object,

an image forming system adapted to project said waves reflected from the object onto said receiver, said image forming system defining an axis, said axis, said transmitter and said receiver defining a plane adapted to pass through said object.

said receiver having at least two receiving zones, at least a part of said receiving zones being disposed in said plane and at least partly offset from said axis, whereby each receiving zone is assigned to a different distance range of said object and delivers a specific output signal when impinged by said short waves reflected from the object, each of said output signals having a characteristic specific for a predetermined distance range,

transducing means arranged within at least a part of said receiving zones and being adapted to produce specific output signals in response to said short electromagnetic waves,

said receiver comprises data storage means being operatively coupled to said transducing means.

data-utilizing means operatively connected with said data storage means and being responsive to the data stored therein,

said data storage means comprises a plurality of bi-stable switching stages,

said specific output signals constitute electric signals, a source of direct current having two terminals,

said switching stage having a first and second switching state and shifting from said first state to said second state when the electric signal exceeds a predetermined threshold level,

a resistor having a plurality of taps, each of which being connected with one of said switching stages, one end of said resistor being connected to said source of direct current, the other end being connected with said data-utilizing means, and

each of said switching stages being operatively connected to disconnect the tap of said resistor associated therewith from one end of said resistor, when said switching stage is in said first state, and to connect the tap associated therewith to said one end of said resistor when said switching stage is in said second state.

2. A rangefinding assembly as set forth in claim 1, in which with all switching stages being in said first state said resistor effecting a maximum resistance associated with largest finite object distances, said transducing means comprising one transducer, which is associated with the switching stage connected to that tap of said resistor, which is next to said other end of said resistor, this one transducer being associated with smallest finite object distances.

3. A rangefinding assembly as set forth in claim 1, said data-utilizing means comprising a resistance measuring device including said resistor and a measuring instrument arranged to indicate the object distance measured last.

4. A rangefinding assembly as set forth in claim 3, said resistance-measuring device consisting of a Wheatstone bridge.

5. A rangefinding assembly as set forth in claim 1, said data-utilizing means comprising a self-balancing bridge including said resistor, a variable resistor controlled by said lens-focusing means and disposed within said self-balancing bridge, said positioning device comprising a positioning motor coupled to said lens-focusing means and responsive to the voltage across the diagonal of said bridge.

6. A rangefinding assembly as set forth in claim 5, comprising amplifier means disposed within the diagonal of said bridge and controlling said positioning motor.

7. A rangefinding assembly within a camera, comprising

a lens structure for said camera,

said lens structure having a variable focus setting,

a variable diaphragm and a range of depth of field depending on said focus setting and said diaphragm,

a transmitter fixed relatively to said camera,

emitting means for emitting short waves,

a receiver mounted in fixed relation with respect to said transmitter, said receiver being adapted to receive short waves emitted by said transmitter and reflected by an object, an image-forming system adapted to project said waves reflected from the object on said receiver, said image forming system defining an axis, said axis, said transmitter and said receiver defining a plane adapted to pass through said object,

said receiver having at least two receiving zones, each of said zones comprising transducing means, said transducing means adapted to produce different output signals with respect to one another when impinged by said short waves,

one of said zones being associated with a distance range of said object lying before the depth of field of said lens structure, the other of said zones being associated with a distance range of said object lying outside said depth of field, and

positioning means connected with said lens structure to vary the distance between said zones in dependence on the variations of the depth of field of said lens structure.

8. A rangefinding assembly as set forth in claim 7, in which said lens structure has a variable focal length with, said range of depth of field depending on said focal length, said transducing means comprising at least two transducers having a variable distance from each other within the plane of the image projected on said receiver, said transducers being at least partly offset from said axis, said positioning means controlling the distance between said two transducers, said distance corresponding to the depth of field of said lens structure.

9. A rangefinding assembly as set forth in claim 7, in which

said lens structure has a variable focal length and is adapted to be adjusted to the setting of values of said focal length and diaphragm which are most critical as to said range of depth of field,

each of said transducing zones being separated so that the extent of said object distance does not exceed said range of depth of field of said lens structure, for a predetermined focus and and setting of said diaphragm.

10. A rangefinding assembly as set forth in claim 7, in which said two specific output signals deriving from said two zones are electric signals, each having a predetermined specific value.

11. A rangefinding assembly as set forth in claim 10, in which one of said values is zero, the other of a predetermined threshold level being different from zero.

12. A rangefinding assembly within a camera, comprising

a lens structure for said camera,

said lens structure having a variable focus setting,

a variable diaphragm and a range of depth of field depending on said focus setting and said diaphragm,

a transmitter fixed relatively to said camera,

emitting means for emitting short waves,

a receiver mounted in fixed relation with respect to said transmitter, said receiver being adapted to receive short waves emitted by said transmitter and reflected by an object, an image-forming system adapted to project said waves reflected from the object on said receiver, said image forming system defining an axis, said axis, said transmitter and said receiver defining a plane adapted to pass through said object,

said receiver having stop members and transducing means,

one of said stop members being associated with a distance range of said object lying before the depth of field of said lens structure, the other of said stop members being associated with a distance range of said object lying outside said depth of field,

positioning means connected with said lens structure to vary the distance between said stop members in dependence on the variations of the depth of field of said lens structure, and

said receiver comprises two stop members movably disposed between said image forming system and said transducing means, said stop members being connected with said positioning means to be moved thereby to positions in which they expose on said transducing means only an area being associated with the range of depth of field.

13. A rangefinding assembly as set forth in claim 12, in which said transducing means consist of a single transducer.