on surface 6 of position resolving photoreceiver 3 swivel arm 8 and thus also deflection mirror 7 are moved far enough in the direction of reception optics 4 to establish a back focal distance s u2032 s1 u2032 s2 u2032 s that corresponds to the encountered object focal distance s if the object focal distance s is shortened as compared to the arrangement shown in fig1 swivel arm 8 and thus also the deflection mirror 7 are moved far enough away from the reception optics 4 until the corresponding now greater back focal distance s u2032 s1 u2032 s2 u2032 has been reached referring to fig3 deflection mirror 7 is mounted on a connecting rod 13 of a four bar linkage the four bar linkage additionally comprises two links 14 and 15 link 14 is connected at one end to a rotary bearing 16 secured in the housing 2 the other end of link 14 is connected to one end of a first upper end of rod 13 with a rotary bearing 17 one end of link 15 is connected to the other lower end of rod 13 with a bearing 18 and a second rotary bearing 19 secures the other end of link.