２０世纪胶片式照相机装备了测光或自动曝光功能后相当长的一段时间内,在使用相机拍摄前首先必须调整并设定好相机的感光度,使之对应于所使用的胶片感光度。这是因为照相机测光系统或自动曝光系统必须以某一个确定的感光度为基准,对被摄物亮度进行测定,以提供正确的曝光数据,而 Twentieth Century film cameras Equipped with metering or auto exposure function for a long period of time before using the camera must first be adjusted and set the camera’s sensitivity, the film corresponds to the sensitivity of the film used. This is because the camera metering system or the automatic exposure system must measure the brightness of a subject with a certain sensitivity to provide the correct exposure data,