Move the part and the receptor and watch magnification, distortion, and unsharpness change — pure geometry, no memorizing.
100 cm SID · 10 cm OID · 0.6 mm focal spot — SOD 90 cm
Magnified 1.11× — a 10 cm object images at 11.1 cm
The beam diverges from a point, so anything held away from the receptor casts a larger shadow. The magnification factor is SID divided by SOD — here 100 ÷ 90. This is size distortion, and it is the reason the affected side goes against the receptor.
Focal-spot blur 0.07 mm
The focal spot is not a point, so each edge of the object is projected from a range of angles and lands as an unsharp band rather than a line. Blur is the effective focal spot times OID over SOD — 0.6 × 10 ÷ 90. Bontrager calls that unsharp edge the penumbra. The small focal spot roughly halves it, which is why it is the one to select when detail matters.
SID is the lever you have left
When OID is fixed by the anatomy, lengthening SID pulls the magnification factor back toward 1 and shrinks the blur with it — both because SOD grows. It is why a chest PA is taken at 180 cm rather than 100. The cost is that receptor exposure falls with the square of the distance, so the mAs has to go up to compensate.
Source: Bushong, Radiologic Science for Technologists, 12e, Ch. 21