are held stationary in the uppermost and lowermost positions respectively against the force of the return spring 37 then when the rear adapter portion 2a of the lens unit is inserted into the bearing portion 1c of the basic unit the horizontal portions of the 34 l 34 shaped grooves 21 receive the respective pins 20 and 20 39 when the shoulder 2b comes to abut on the front edge of the basic unit housing 1 the operator then needs to push down the control knob 22 to take the click stop means 24a and 31a out of action whereby the follower plates 25 and 25 39 are moved in the reversed directions to those described above under the action of the spring 37 thus the first support pin 20 moves downwards in the vertical portion 21b of the 34 l 34 shaped groove 21 while the 2nd support pin 20 39 moves upwards in the vertical portion 21b 39 of the 34 l 34 shaped groove 21 39 and they are stopped in certain prescribed positions at this time the spring 28 and 28 39 press the support pins 20 and 20 39 against the rear side edges of the slots 38 39 the adapter portion 2a of the lens unit is light tightly fitted in the stepped.