the cam groove 31 for the second group 39 s rear unit because the distance lever 38 acts as a key and the cam pin 30 for the second group 39 s rear unit moves by a given distance along the optical axis direction under the action of both cam grooves the second group cam groove 25 and the cam groove 31 for the second group 39 s rear unit as the cam pin 30 for the second group 39 s rear unit moves in the optical axis direction the frame 29 for the second group 39 s rear unit planted in it moves together with the cam groove 34 for the second group 39 s front unit from a broken line position to a solid line position thus the cam pin 33 for the second group 39 s front unit located at the position of intersection of the key groove 35 for the second group 39 s front unit and the cam groove 34 for the second group 39 s front unit provided in the fixed frame 11 too moves the same distance in the same direction as the cam pin 30 for the second group 39 s rear unit consequently the front and rear units of the second group the frames 32 and 29 for the second group 39 s front and rear frames while they are kept at a given distance move by a given distance in the optical axis direction under the.