1. A blur detecting device for a camera, comprising: a sensor having a function of photo-electric conversion upon receiving light from an object; timing means for controlling output timing of said sensor at a first point of time and a second point of time in a photographing operation of the camera; means for sampling an output signal of said sensor by a first threshold level at said first point of time and for sampling the output signal of said sensor by a second threshold level at said second point of time; means for determining said first and second threshold levels of said sampling means based on information on a preset or computed shutter speed and/or a preset or computed aperture value; and means for detecting an amount of blur in an image by outputs of said sampling means at said first point of time and said second point of time.