A Gauss type standard lens for SLR cameras, developed based on the optical theory of S (screw) mount 50mm f/1.8 lens. This lens is applied as a standard lens for Canon SLR cameras from FX to TL, having automatic diaphragm control at full aperture mechanism in it. The automatic diaphragm mechanism remains fully open during focusing and composition to provide a bright viewfinder image, but must be stopped down for metering. It automatically closes down to the aperture setting when shutter button is pressed and automatically opens up again when the exposure is completed. This lens uses mechanical linkage for controlling this automatic diaphragm operation.