SID must be increased by 42 inches , or 3.5 feet --
As OID is increased, recorded detail is diminished as a result of magnification distortion. If the OID cannot be minimized, an increase in SID is required to reduce the effect of magnification distortion. However, the relationship between OID and SID is not an equal relationship.
In fact, to compensate for every 1 inch of OID, an increase of 7 inches of SID is required. Therefore, an OID of 6 inches requires an SID increase of 42 inches. That is why, a chest radiograph with a 6-inches air gap is usually performed at a 10-foot SID.