divided into right and left hand parts with a transparent part 7b intervening therebetween in this case the light total reflectedly from the plane mirror 12 passes through the intermediate or transparent part 7b to reach the light sensor for focusing adjustment by providing such a transparent part 7b the part of light split off by the half mirror surface 7a is not reflected again by the half mirror surface 7a and reaches the light sensor 9 for focusing adjustment very efficiently therefore as compared with the case of fig5 a and 5 b even an object of lower brightness level can be detected fig7 illustrates an example of using a concave mirror for reflecting the light split by the beam splitting member wherein the same reference numerals denote the similar parts to those shown in fig4 the half mirror block 7 having formed therein a half mirror surface 7a inclined 45 u00b0 with the optical axis of the photographic system is arranged in an afocal space between the compensator 3 and the diaghram 5 the light split off by the half mirror mirror 7a is totally reflected by a concave mirror 13 then while converging transmitted backward and then passed again through the half mirror block 7 reaching the light sensor 9 for focusing adjustment fig8 illustrates another embodiment of the invention in this case the concave mirror 13 of fig7 is unified with the half mirror block 7 to form a special half mirror block 7 39 in other words.