Final answer:
A focal-spot size of 0.3mm or smaller is essential for magnification radiography to achieve higher resolution and detailed images, which allows for the visualization of very small structures.
Step-by-step explanation:
A focal-spot size of 0.3mm or smaller is essential for magnification radiography. Magnification radiography is a specialized form of radiographic imaging, where a small focal-spot size is necessary to enhance image resolution and detail. It allows for the visualization of very small structures that are not easily seen on standard radiographs. This type of imaging is beneficial when higher resolution details are required for diagnosis. Techniques like computed tomography (CT) and single-photon-emission computed tomography (SPECT) are different forms of imaging that can provide detailed images, but they do not rely on the same focal-spot size requirements as magnification radiography does.