Final answer:
The factor that directly controls subject contrast in radiography is kVp (Kilovolt peak), which adjusts the energy and penetration of the x-ray beam to affect tissue differentiation on the image.
Step-by-step explanation:
The factor that directly controls subject contrast in radiography is kVp (Kilovolt peak). The kVp is the peak voltage applied to the x-ray tube, which affects the energy and penetration of the x-ray beam. A higher kVp results in x-rays with higher energy that can penetrate more deeply through different types of tissues, thereby reducing the differences in the absorption of the x-ray photons. This leads to lower subject contrast as it makes it harder to distinguish between tissues with similar densities. Conversely, a lower kVp will result in higher subject contrast as the x-rays have less energy and tissues will have more distinct differences in absorption levels. Therefore, kVp is the primary control factor for managing how well different tissues are delineated in an x-ray image.