The present invention relates to an X-ray diagnostic apparatus capable of converting an X-ray image formed by radiating X rays through a patient, into a video signal. 
One type of diagnostic apparatus is an X-ray diagnostic apparatus wherein an X-ray image formed by irradiating X rays through a patient to be diagnosed is converted to an optical image by an image intensifier tube and wherein the optical image is then converted to a video signal by a television camera.  The image pick-up quality of this apparatus depends on the amount of X-rays irradiated, conversion characteristic of the image intensifier tube, diaphragm of the television camera and so on.  Conventionally, the optimum pick-up quality is determined by adjusting the amount of X rays irradiated.  More specifically, pre-irradiation of X rays is repeated several times while increasing the amount of X rays.  When the average value of the video signal output from the television camera reaches a value which is in the center of the dynamic range of the video signal of the camera, image pick-up conditions are regarded at this instant as being optimum, and an image forming substance is then injected into the blood stream of the patient, and main irradiation of X rays is carried out under this optimum condition to obtain a video signal which represents the image of the patient. 
Image pick-up conditions determined by this process, however, may not always be optimum.  The dynamic range of the television camera is narrow relative to incident light, and when the intensity of incident light becomes larger than a certain value, a current value of the output signal (video signal) is saturated.  Therefore, the brightest portion is excluded from the dynamic range of the camera under the conventional image pick-up condition and becomes white in the image thus obtained. 
In addition, the conventional optimum condition is obtained after repeating pre-irradiation of X rays several times.  Even though the amount of X rays irradiated one time is small, it becomes larger till main irradiation of X rays is finished, and this is not desirable.