What is the relationship between the focus, limb and film of X-ray tube?

According to the geometric projection principle of X-ray money, in X-ray projection, the focus of X-ray tube and the center of film should be on the same straight line, and the center line should be perpendicular to the measured object and colloid. The irradiation field may include only the part to be inspected. The relationship between them is: p is the size of the blur penumbra, d is the object distance, d is the focal length, and f is the size of the effective focus area.

As can be seen from the formula, the greater the distance between focal films, the more parallel the light shining on the film, the smaller the projection magnification of the object, the clearer the image, the smaller the distance between the object and the film, and the smaller the penumbra, the clearer the image. Therefore, in X-ray photography, limbs and lesions should be as close as possible to the film, and the distance between focal films is generally 75 ~ 100 cm. In order to reduce the distortion of heart enlargement, 180 ~ 200 cm long-distance projection should be adopted in cardiac photography.

When the object distance increases, the image is magnified, and its magnification is about equal to the ratio of focal length to focal object distance, which is the principle of magnifying photography.