The reason for this distinction is that dense body parts such as bone have little film density, while air in lung is not a dense part of the body, yet produces an area of high film density. The contrast of the radiographic image can be defined as the difference in opacity between 2 regions of the radiograph. The highest or ultimate contrast is when an image is either black or white. Radiographs are not made to yield the ultimate contrast, as we would not be able to differentiate tissues of similar density. Radiographs with high contrast have fewer shades of gray, while or high latitude radiographs have greater numbers of shades of gray.
Radiology is inherently an imaging system with low contrast resolution.
It is not very efficient in separating object of similar characteristics. In radiology, all soft tissues look the same and one cannot differentiate between liquid and tissues. For example, the urinary bladder wall cannot be differentiated from the urine and the cardiac muscle from the blood. These structures will appear on the radiograph as a soft tissue opacity. In addition, scatter radiation adds a uniform fog on the radiographic image.