Final answer:
A long scale of contrast in a radiograph is achieved by using lower kV settings on the X-ray machine, creating a subtle distinction between shades of gray due to a wider range of X-ray photon penetration through different tissue densities.
Step-by-step explanation:
The question pertains to radiographic imaging, specifically the factors influencing the scale of contrast in a radiograph. In radiology, a long scale of contrast refers to an image that displays a large range of densities, meaning there is a subtle distinction between the shades of gray. This is typically achieved by using lower kV settings on the X-ray machine, allowing a greater range of X-ray photons to pass through various densities. The lower energy setting results in more gradations in tissue density being represented in the radiograph, which is important for examinations like a chest X-ray where subtle differences in tissue must be distinguished. In contrast, higher kV settings result in short scale contrast, which is better for detecting significant differences, such as in bone fractures where a clear distinction between the bone and surrounding tissue is necessary.