PURPOSE: To adjust the quantity of light more accurately when a stop is almost closed by slanting the lengthwise center line of the guide pin oblong hole of a stop link lever at a constant angle to a straight line which connect the center of the stop and the center of rotation of a turning pin to each other.

CONSTITUTION: When the turning pin 12 moves from an open aperture state up to an angle α1 which is a half as large as a turnable angle, the stop link lever 13 turns from a point A to a point B and the angle of movement becomes β1. When the pin moves by an angle α2(α1=α2), the stop link lever 13 turns from the point B to a point C by an angle β2. In this case, β1>β2. Similarly, when the pin 12 turns by α2/2 from the state shown in a figure (b) during the movement from the figure (b) to a figure (c), the turning angle of the stop link lever 13 is larger when the pin 12 further rotates by α2/2. Namely, variation in stop area to the turning angle of the turning pin 12 decreases as the stop is closed more as more to zero.