[1] Motion unsharpness is caused by movement of the patient, the detector or the source of X-rays, during the exposure.
In computed radiography systems it the size of the laser used to read the phosphor plate in the cassette reader.
Fine focal spot sizes will minimise geometric unsharpness, and therefore give more detailed images, but it is often impossible to employ them due to the tube loading necessary in the exposure.
Keeping the ratio FFD:OFD high will minimise geometric unsharpness.
If this is not possible however, then increasing FFD beyond the normal 100–110 cm will be necessary to keep the unsharpness level acceptable.