The origin or focus of an X-ray beam used for taking diagnostic radiographs should ideally be a point source but at the same time if it is to sustain the heat produced it should ideally be a point source but at the same time if it is to sustain the heat produced it should be as large as possible and so a compromise has to be reached.

This is achieved by a simple but ingenious idea devised by Goetze, termed line focus. The significant feature of this is that the face of the anode bearing the target is usually angled at 22.5˚ from the vertical. As a result of this the actual focus or target receiving the bombardment can be rectangular and some three times the area of the effective focus from which the beam originated.

In diagnostic X-ray tubes the actual size of the effective focus is likely to be within the range 0.6-105 mm2. Some tubes provide a choice of two sizes, the smaller or fine focus being employed when fine detail is required in the radiograph and when the output of the machine can be restricted, while the larger or coarse focus is used when it necessary to subject the tube to heavier current.

A further way of extending the area of the actual focus without affecting the effective focus is by means of a rotating anode which will be found in the larger and more powerful type of X-ray tube and apparatus. Here the target is situated on a flange at the edge of a wheel-like structure which rotates at high speed during the exposure and so expands the area subjected to bombardment by several hundred times.