Light absorbed in a substance is a fixed ratio of incident radiation. Film blackening is proportional to this. Students of Calculus use incremental steps in their analysis. All entirely normal anatomy within the patient's chest is soft tissue density and either contains some air or lies adjacent to some air. Any denser object will exceed the greyscale perception threshold and always be visible.
Calcification is an example.
Felsen's original example was to look at the image resulting from the passage of X-rays through boxes, which contained wedges of wax and added mineral oil. He attempted to demonstrate the low contrast densities lying adjacent or projected by each other. The image shows a ground plan of the experiment alongside its projection. Tuddenham made a slightly different model with curved surfaces . In fact, the geometry is more complex, since an xray is generated effectively from a Point source.