The brightness of an X-ray video image during fluorography is maintained at a substantially constant level by a control circuit which varies the X-ray dose in relation to changes in the average brightness of the X-ray image. Selected picture elements of the image which lie within a defined region in the image and which have a brightness above a given level are used to derive the average X-ray image brightness. The brightness of the selected picture elements are summed and the number of such picture elements counted. The summation of the brightness and the picture element count are both scaled by a factor of 2 n to reduce the size of the numbers and the complexity of the averaging circuitry. The scaled brightness summation is divided by the scaled picture element count and the result is stored as the average X-ray image brightness.