embodiments of the present invention will be described in conjunction with the drawings . the structure of binoculars 1 , shown as a telescope in fig2 - 14 , in accordance with the first embodiment of the present invention , will be described using fig1 and fig2 . fig1 is a perspective view showing the appearance of the binoculars 1 of the first embodiment of the present invention . fig2 is an exploded oblique view of a lens frame drive and barrier mechanism incorporated in the binoculars 1 . in the description to be made below , an optical axis o is the optical axis of an observation optical system included in the binoculars , and z directions are directions parallel to the optical axis o . a direction z 0 out of the z directions is a direction towards an eyepiece , and a direction z 1 is a direction towards an objective . directions x are rightward and leftward directions orthogonal to the optical axis o . a direction of rotation d 0 is a clockwise direction relative to the eyepiece , and a direction of rotation d 1 is a counterclockwise direction relative to an observer ( see fig2 ). these reference numerals will also be applied to subsequent embodiments . the binoculars 1 of the present embodiment have , as shown in fig1 and fig2 a body 2 , a lens barrel bearing frame 3 , a lens barrel bearing frame 4 , lens frames 31 and 33 , lens frames 32 and 34 , left and right barrier mechanisms 6 and 7 , a lens frame drive 20 , and a lens frame handling unit 16 . the lens barrel bearing frame 3 is supported to be able to slide leftward relative to the body 2 , and slid for accomodating a user &# 39 ; s pupil distance . the lens barrel bearing frame 4 is fixed to the body 2 . the lens frames 31 and 33 holding a left objective and eyepiece 25 and 26 serving as an observation optical system are borne by the lens barrel bearing frame 3 so that the lens frames can advance or withdraw freely . the lens frames 32 and 34 holding a right objective and eyepiece 27 and 28 are borne by the lens barrel bearing frame 4 . the left and right barrier mechanisms 6 and 7 include left and right lens barriers 8 and 9 that are barrier members . the lens frame drive 20 drives the lens frames , which are optical system thrusting means , so that the lens frames will advance or withdraw . the lens frame handling unit 16 drives the lens frame drive 20 so as to focus the lenses and open or close the barriers . the barrier mechanism 6 as shown in fig2 consists of a slide base 10 , a spring 29 , a lens barrier 8 , and a torsion spring 12 . the slide base 10 that is a sliding member is supported by a guide groove 3 b bored in a stowage chamber 3 a of the lens barrel bearing frame 3 so that the slide base 10 can freely slide in the z directions . the spring 29 serving as a second constraining means constrains the slide base 10 to move in the direction z 0 . the lens barrier 8 is supported by a support shaft 11 at the distal end of the slide base 10 so that the lens barrier 8 can pivot freely . the torsion spring 12 serving as a first constraining means constrains the lens barrier 8 to pivot in a direction in which the objective is protected ( shielded ). the lens barrier 8 is a member movable to a position of protection ( position of blocking ) p 8 a at which it protects the objective or a position of withdrawal ( position of freeing ) p 8 b at which it has withdrawn into the stowage chamber 3 a . the position of withdrawal p 8 b is a position at which the lens barrier 8 has withdrawn from the front surface of the objective , due to a sliding movement in the direction z 0 made by the slide base 10 , after pivoting in a direction of freeing . a manipulation knob 10 a is formed on the lower side of the slide base 10 . the barrier mechanism 7 has the same structure as the barrier mechanism 6 . the barrier mechanism 7 consists of a slide base 13 , a spring , a lens barrier 9 , and a torsion spring 15 . the slide base 13 is supported to be able to freely slide . the spring serving as a second constraining means that is not shown constrains the slide base 13 to move in the direction z 0 . the lens barrier 9 is supported by a support shaft 14 so that it can rotate freely . the torsion spring 15 serving as a first constraining means constrains the lens barrier 9 to move in the direction of blocking . the lens frame handling unit 16 consists of a focusing ring 17 , a cam unit , and a support shaft 18 . the focusing ring 17 can be manipulated externally . the cam unit is united with the focusing ring 17 and has a cam groove 17 a that serves as an optical system thrusting means . the support shaft 18 is borne by the body 2 so that it can rotate freely , and supports the focusing ring 17 and cam unit . the lens frame drive 20 has a drive base 21 , a slide plate 22 , and a support plate 23 . the drive base 21 has a slave pin 21 a and a guide groove 21 c , and is borne by the body 2 so that it can slide freely in the z directions over a distance defined by guide pins 2 b and 2 c . the slave pin 21 a is engaged with - the cam groove 17 a so that it can slide freely . the slide plate 22 is borne by a guide groove 21 b in the - drive base 21 so that it can slide freely in the x directions . the slide plate 22 can freely slide in the z directions in the lens barrel bearing frame 3 over a distance defined by a guide groove 22 a while being interlocked with the drive base 21 . the support - plate 23 is fixed to the drive base 21 and bears the right lens frame 32 so that the right lens frame can slide freely . a lens frame support 24 is fixed to the lens frame 31 holding the left objective . a projection 31 a is projecting from the lower part of the lens frame 31 . the lens frame support 24 is always engaged with an engagement groove 22 b bored in the distal part of the slide plate 22 . the lens frame support 24 moves together with the slide plate , whereby the lens frame 31 is driven to advance or withdraw in the z directions . when the lens frame 31 is thrust in the direction z 1 , the projection 31 a presses the inner surface of the lens barrier 8 located at the position of blocking , and thus causes the lens barrier 8 to fall down in the direction of freeing . the lens frame 32 holding the right objective has the same structure as the lens frame 31 . the lens frame 32 is driven to advance or withdraw by means of the support plate 23 . a projection of the lens frame 32 , which is not shown , causes the lens barrier 9 to fall down in the direction of freeing . actions made in the binoculars 1 of the first embodiment of the present invention , which have the foregoing structure , will be described in conjunction with fig3 to fig5 . fig3 is a longitudinal sectional view of the left lens frame and its surroundings in a lens - protected ( shielded ) state equivalent to the stowed state of the binoculars . fig4 is a longitudinal sectional view of the left lens frame and its surroundings in a lens - freed ( barrier withdrawn ) state equivalent to the usable state of the binoculars . fig5 graphically shows a change of the lens barrier from the closed state to the open state thereof and a change in the position in a direction of thrust of a lens in relation to the angle of rotation θ at which the focusing ring is positioned . the relationship shown in fig5 is applicatable to the other embodiments described later . as shown in fig5 when the binoculars 1 are stowed , the focusing ring 17 is positioned at an angle of rotation θ 0 . the lens frames 31 and 32 are plunged into their positions of collapse by means of the cam groove 17 a and lens frame drive 20 . at this time , the slide bases 10 have slid in the direction z 1 because the knobs 10 a were manipulated . as shown in fig3 the lens barrier 8 is located at the position of blocking p 8 a , and the lens barrier 9 is located at the position of blocking p 9 a ( see fig1 ). for making the binoculars 1 usable , the focusing ring 17 is rotated in the direction d 1 and positioned at an angle of rotation θ 1 . with the rotation , the drive base 21 and slide plate 22 are moved in the direction z 1 . the lens frame 31 is thrust to the in - focus position for infinite focus viewing . before the lens frame reaches the in - focus position for infinite focus viewing , that is , before the focusing ring 17 is positioned at the angle of rotation θ 1 , the projection 31 a of the lens frame presses the lens barrier 8 in the direction z 1 as shown in fig4 . when the lens barrier is tilted by a predetermined angle to fall down , the constraining force produced by the spring 29 for constraining the slide base 10 to move in a certain direction operates on the slide base . the lens barrier 8 is then pulled into the stowage chamber 3 a in the lower part of the left lens barrel bearing frame 3 along the guide groove 3 b together with the slide base 10 . in this state , the lens barrier 8 has moved to the position of withdrawal p 8 b . furthermore , the lens barrier 9 has also moved to the position of withdrawal . the binoculars 1 are thus put to an observation ( discernment )- enabled state ( see fig5 ). in this state , the focusing ring 17 is further rotated in the direction d 1 to an angle θ 2 . the lens frames 31 and 32 are thrust to their in - focus positions for the closest focus viewing . for adjusting the binoculars at accommodate user &# 39 ; s pupil distance , the left lens frame bearing frame 3 is slid in an outward x direction . with the sliding manipulation , the slide plate 22 is moved together with the left lens frame bearing frame 3 and lens frame 31 in the x direction by means of the guide pin 3 c . consequently , the pupil distance can be modified . for returning the binoculars 1 from the usable state to the stowed state , first , the binoculars are adjusted to the narrowest distance . however , this manipulation is not required . the pupil distance may remain wide . thereafter , the focusing ring 17 is rotated in the direction d 0 to the angle θ 0 . the lens frames 31 and 32 are thus plunged into the positions of collapse . the knobs 10 a are manipulated in order to slide the slide bases 10 in the direction z 1 . this causes , as shown in fig3 the lens barriers 8 and 9 to pivot to the positions of blocking p 8 a and p 9 a ( see fig1 ). according to the binoculars 1 of the first embodiment of the present invention , the manipulation of freeing to be performed for withdrawing the lens barriers can be interlocked with a manipulation to be performed for rotating the focusing ring for focusing the lenses . the binoculars can therefore be quickly brought to the observation - enabled state . moreover , as mentioned above , driving the lens barriers for freeing the objectives may be interlocked with movements of the lens frames . this results in a simple barrier mechanism . next , the structure of binoculars 40 will be described as that of a telescope in accordance with the second embodiment of the present invention in conjunction with fig6 . fig6 is an exploded oblique view showing a lens frame drive and barrier mechanism incorporated in the binoculars . in the binoculars 1 of the first embodiment , for returning the lens barrier 8 to the position of lens protection ( position of blocking ) p 8 a , the manipulation knob 10 a of the slide base 10 must be manipulated . this may be annoying . in the binoculars 40 of the present embodiment , an action to be made for returning the lens barrier 8 to the position of lens protection ( position of blocking ) p 8 a is interlocked with an action to be made for plunging the lens frame 31 into the position of collapse ( position of stowage ). the binoculars 40 have the same components as the binoculars 1 of the first embodiment except a barrier interlocking mechanism . only the different component will be described . the same reference numerals will be assigned to the same members . a barrier mechanism 45 for a left objective included in the binoculars 40 has a slave pin 43 fixed to the slide base 10 . the knob 10 a is excluded . moreover , a support shaft 3 d is fixed to the left lens barrel bearing frame 3 . a slide base drive lever 42 that is a rotating member for pressing the slave pin 43 and thus driving it is supported by the support shaft 3 d so that the slide base drive lever can freely swivel . restriction pins 44 a and 44 b for limiting the swiveling of the drive lever 42 to a required range are placed near the drive lever , as shown in fig7 . the lens frame support 24 can abut on the slide base drive lever 42 . when the lens frame support 24 moves in the direction z 0 that is a direction in which the lens frame is stowed , the drive lever 42 is driven to swivel clockwise . this causes the slide base 10 to slide in the direction z 1 via the slave pin 43 . with the movement of the slide base 10 , the lens barrier 8 is pushed to a position at which the lens barrier can pivot in a direction in which the objective is shielded . the constraining force produced by the torsion spring 12 causes the lens barrier to pivot to the position of protection p 8 a . a barrier mechanism for the right objective is not shown but has the same components as the barrier mechanism 45 for the left objective . actions to be made for opening or closing the barriers included in the binoculars 40 of the second embodiment having the foregoing components will be described in conjunction with fig7 to fig9 . fig7 is a plan view showing the interior of the binoculars 40 that is stowed with the lens frames moved to their positions of collapse . fig8 is a plan view showing the interior of the binoculars 40 that is usable with the lens frames moved to their in - focus positions for infinite focus viewing . fig9 is a plan view showing the interior of the binoculars 40 that is usable with the lens frames moved to their in - focus positions for the closest focus viewing . the relationship among the angle of rotation at which the focusing ring is positioned , the position of thrust to which the objective is thrust , and the movement of the barrier is identical to that graphically shown in fig5 . as shown in fig7 when the binoculars 40 are stowed , the focusing ring 17 is positioned at its position of stowage . the lens frames 31 and 32 are plunged into their positions of collapse . moreover , the lens barrier 8 has pivoted in a direction of closing due to the constraining force produced by the torsion spring 12 , and is retained at the position of blocking p 8 a . at this time , the slide base drive lever 42 has been pressed by the lens frame support 24 and rotated clockwise . for changing the binoculars 40 from the stowed state to the usable state , similarly to that in the first embodiment , the focusing ring 17 is rotated in the direction d 1 and positioned at the angle of rotation θ 1 . this causes the lens frame 31 to thrust to its in - focus position for infinite focus viewing as shown in fig8 . immediately before the lens frame reaches the in - focus position for infinite focus viewing the lens barrier 8 is pressed by the lens frame 31 and moved to the position of withdrawal p 8 b . the front surface of the objective 25 is thus freed . at this time , the slide base 10 moves together with the lens barrier 8 in the direction z 0 to its position of withdrawal in the withdrawal chamber . the slide base drive lever 42 rotates counterclockwise . in the above state , the focusing ring 17 is further rotated in the direction d 1 and positioned at the angle θ 2 . consequently , as shown in fig9 the lens frames 31 and 32 are thrust to their in - focus positions for the closest focus viewing in the manner as they are in the first embodiment . for returning the binoculars 40 from the usable state to the stowed state , similarly to that in the first embodiment , the focusing ring 17 is rotated in the direction d 0 and positioned at the angle of rotation θ 0 . this causes the lens frames 31 and 32 to plunge into their positions of collapse . along with the plunging action , as shown in fig7 the lens frame support 24 withdrawing in the direction z 0 drives the slide base drive lever 42 to swivel clockwise . with the swiveling , the drive pin 43 presses the slide base 10 . the slide base 10 then moves in the direction z 1 . along with the movement of the slide base 10 , the lens barrier 8 moves outward from the stowage chamber 3 a of the lens barrel bearing frame 3 . the lens barrier 8 then pivots due to the constraining force produced by the torsion spring 12 and lies at the position of blocking p 8 a . the slide base moves together with the lens barrel in the direction z 1 . in the binoculars 40 of the second embodiment , driving the lens barrier to withdraw it for freeing the objective and driving the lens barrier to the position of blocking can be initiated by manipulating the focusing ring , which is used for focusing the binoculars , to rotate it . manipulating the binoculars to change their states from the observation - enabled state to the stowed state or vice versa can be achieved very easily . next , the structure of binoculars 50 will be described as that of a telescope in accordance with the third embodiment of the present invention in conjunction with fig1 . fig1 is an exploded oblique view of a lens frame drive and barrier mechanism incorporated in the binoculars . in the binoculars 1 of the first embodiment , the slide base 10 is needed for plunging the lens barrier 8 into the position of withdrawal p 8 b , and the stowage chamber 3 a is needed for stowing the lens barrier and slide base . this may cause the lens barrel bearing frame to become complex or large - sized . in the binoculars 50 of the present embodiment , neither the slide base 10 nor chamber 3 a is needed . the binoculars 50 have the same components as the binoculars 1 of the first embodiment except the barrier mechanism . only the different component will be described . the same reference numerals will be assigned to the same members . a barrier mechanism 55 for the left objective of the binoculars 50 has a lens barrier 58 supported by a support shaft 11 fixed to the left lens barrel bearing frame 3 so that the lens barrier 58 can pivot freely . the lens barrier 58 has a claw 58 a extending from an upper position on the lens - side surface thereof . the lens barrier 58 is constrained to move in a direction of freeing by means of a torsion spring 54 serving as a first constraining means . a barrier locking lever 52 that is a rotational interlocked member has a lock claw 52 a that can be engaged with the claw 58 a . the barrier locking lever 52 is attached to the support shaft 3 e of the left lens barrel bearing frame 3 . a spring 53 that is a second constraining means for constraining the barrier locking lever 52 to move in a direction of engagement is coupled to the barrier locking lever 52 . furthermore , a pin 31 b capable of pressing the barrier locking lever 52 in a direction of unlocking is fixed to the left lens frame 31 . the barrier mechanism for the right objective has the same components as the barrier mechanism 55 for the left objective , though it is not illustrated . actions to be made for opening or closing the barrier in the binoculars 50 of the third embodiment having the foregoing components will be described in conjunction with fig1 to fig1 . fig1 is a plan view showing the interior of the binoculars 50 that are stowed . fig1 are a longitudinal sectional view of a lens frame and its surroundings in the state shown in fig1 . fig1 is a plan view showing the interior of the binoculars 50 that are usable with the lens frames moved to their in - focus positions for infinite focus viewing . fig1 is a longitudinal sectional view of a lens frame and its surroundings in the state shown in fig1 . the relationship among the angle of rotation at which the focusing ring is positioned , the position of thrust to which the objective is thrust , and the movement made by the barrier shall be identical to the relationship graphically shown in fig5 . as shown in fig1 and fig1 , when the binoculars 50 are stowed , the focusing ring 17 are positioned at the angle of rotation θ 0 to its position of stowage . the lens frame 31 has been plunged to its position of collapse . moreover , the lens barrier 58 has the claw 58 a thereof locked by the barrier locking lever 52 , and is retained at its position of blocking p 58 a . for changing the binoculars 50 from the stowed state to the usable state , similarly to that in the first embodiment , the focusing ring 17 is rotated in the direction d 1 and positioned at the angle of rotation θ 1 . the lens frame 31 is thus thrust to its in - focus position for infinite focus viewing . immediately before the lens frame reaches the in - focus position for infinite focus viewing , as shown in fig1 , the pin 31 b presses the barrier locking lever 52 along with the movement of thrusting made by the lens frame 31 . the claw 58 a of the lens barrier 58 is then unlocked . the lens barrier 58 pivots due to the constraining force produced by the torsion spring 54 from its position of blocking p 58 a ( fig1 and 13 ) through a middle position p 58 c ( fig1 ) to , as shown in fig1 , its position of withdrawal p 58 b . consequently , the front surface of the objective 25 is freed . the focusing ring 17 is further rotated in the direction d 1 and positioned at the angle θ 2 . the lens frame 31 is then thrust to its in - focus position for the closest focus viewing in the same manner as that in the first embodiment . for returning the binoculars 50 from the usable state to the stowed position , the focusing ring 17 is rotated in the direction d 0 and positioned at the angle of rotation θ 0 in the same manner as that in the first embodiment . the lens frame 31 is then plunged into its position of collapse . the pin 31 b of the lens frame 31 moves in the direction z 0 . this causes the barrier locking lever 52 to pivot counterclockwise up to a position at which it is engaged with the locked claw 58 a of the lens barrier 58 . the lens barrier 58 is manipulated so that it will pivot to its position of blocking p 58 a in fig1 against the constraining force produced by the torsion spring 54 . the claw 58 a of the lens barrier 58 is locked by the barrier locking lever 52 . the lens barrier 58 is retained at the position of blocking p 58 a , and the binoculars 50 are put in the stowed state . in the foregoing binoculars of the third embodiment , driving the lens barrier to withdraw it for freeing the objective can be achieved by rotating the focusing ring , which is used to focus the binoculars . moreover , the stowage chamber in which the lens barrier 58 is stowed is unnecessary . this leads to a simplified barrier mechanism . consequently , a compact lens frame can be realized . as mentioned in the telescopes of the embodiments of the present invention , the barrier member is moved from its position of protection ( position of shielding ) at which it protects an optical system to its position of withdrawal ( position of freeing ) responsively to an action made by the optical system thrusting means . this leads to a simplified manipulation changing the binoculars to a usable state permitting observation . consequently , the binoculars can be handled easily .