What happens to a radiographic image when the object film distance is increased?

Answer 1

When the object to image distance (OID) is increased, it creates what is called an "air-gap". This space between object and film reduces the amount of scatter radiation that reaches the film by allowing divergent rays travelling at steep angles to miss the x-ray film or cassette. Though it is not the same principle as using a grid-cap over a cassette, it does achieve similar results. A grid uses lead strips to absorb scatter x-ray photons while an air-gap simply provides the scatter x-rays enough space to pass by the film. These are primarily low-energy photons that would only add density to your images w/out diagnostic benefit. Increasing the OID also has two other important effects on the radiographic image: it results in magnification (a form of geometric distortion) and a loss of detail due to an larger pnenumbra. This can be demonstrated by pointing a desk lamp at a wall and making a "Shadow Puppet" using any object and observing the change in the cast shadow when you increase or decrease the distance between the object and the wall (the shadow is analogous to the radiographic image...)