Why should the focal length be increased in chest X-ray photography?

In chest X-ray photography, the reason to increase the focal length is to reduce the magnification distortion of the image and increase the photosensitive effect.

According to the optical principle, the change of focal length will affect the magnification of the image. When the focal length decreases, the magnification of the image increases and the image appears larger; However, when the focal length increases, the magnification of the image decreases and the image becomes smaller. In the specific photography operation, increasing the focal length can effectively reduce the magnification distortion of the image.

In addition, an important consideration is the photosensitive effect. The photosensitive effect is inversely proportional to the square of the focal length, that is, the focal length decreases and the photosensitive effect increases; When the focal length increases, the photosensitive effect decreases. Therefore, increasing the focal length in the process of photography can reduce the amplification distortion of the image, but it will also lead to the reduction of the photosensitive effect.

Generally speaking, increasing the focal length is mainly to reduce the distortion of image amplification, but in practice, it is necessary to find a balance between reducing the distortion of image amplification and increasing the photosensitive effect.