CoreFull access
SID, SOD and OID
SID = SOD + OID. Know two of them and you can find the third, then the magnification.
The three distances
Three distances describe the geometry of every exposure. SID, the source-to-image-receptor distance, runs from the focal spot to the image receptor. SOD, the source-to-object distance, runs from the focal spot to the part. OID, the object-to-image-receptor distance, runs from the part to the receptor. The part sits between the tube and the receptor, so SID always equals SOD plus OID.