When the foot is placed directly on the image receptor, there is no OID.

Question

Quizplus · Accepted Answer

OID, or Object-to-Image Distance, is always present unless the object being imaged is in direct contact with the image receptor. In the case of a foot X-ray, even if the foot is placed directly on the image receptor, there is still a minimal OID due to the three-dimensional shape of the foot, which prevents certain parts (like the arch) from being in direct contact with the receptor.

When the Foot Is Placed Directly on the Image Receptor