What is the formula for calculating the source-to-object distance (SOD)?

What is the formula for calculating the source-to-object distance (SOD)?

• A. Source-to-image distance (SID) minus object-to-image distance (OID)
• B. Object-to-image distance (OID) minus SID
• C. SID plus OID
• D. OID

Answer: (A)

The distance from the source to the object is what remains when you subtract the distance from the object to the receptor from the total distance from source to receptor. In other words, source-to-object distance equals source-to-image distance minus object-to-image distance. So SOD = SID − OID. This makes sense because the object sits between the source and the receptor, so the segment from the source to the object is the remaining portion of the total source-to-image path. If OID increases, SOD decreases (for a fixed SID). The other expressions don’t align with the geometry of the setup.

The distance from the source to the object is what remains when you subtract the distance from the object to the receptor from the total distance from source to receptor. In other words, source-to-object distance equals source-to-image distance minus object-to-image distance. So SOD = SID − OID. This makes sense because the object sits between the source and the receptor, so the segment from the source to the object is the remaining portion of the total source-to-image path. If OID increases, SOD decreases (for a fixed SID). The other expressions don’t align with the geometry of the setup.