A camera system with function of automatically coordinating flash intensity includes a lens, an image sensor, a focus system, a focus sensor and a control unit. The lens controls light amount that entrance into camera. The focusing system drives the lens to focus thereby forming a clear image on the image sensor. The focus sensor senses first step information of the focusing system during the focusing process, and sending the information to a control unit. The control unit transforms the result of the focusing system to a distance that the object apart from the camera, and configures out the guide number with the aperture number thereby calculating the flash intensity. The present camera systems may measure the distance that the objects apart from the camera, and low down the cost and power consumption. The camera systems may also precisely control the flash intensity thereby coordinating the exposure effect.