In projection radiography, X-rays originate from a small region on the anode called the focal spot. From this point, the photons diverge outward in all directions as they travel toward the patient and the image receptor. The distance from this source to the detector is the source-to-image distance.

Because the X-ray beam diverges, the distance between the source and the detector affects the geometric projection of the anatomy onto the image receptor. When the detector is closer to the source (short SID), the diverging beam spreads more rapidly across the image plane. This increases image magnification and geometric distortion.

Increasing SID reduces this divergence effect and therefore reduces magnification.

The relationship between SID and radiation intensity is described by the inverse square law:

I ∝ 1 / d²​

where I is beam intensity and d is the distance from the X-ray source.

This means that if the distance from the source is doubled, the intensity of the X-ray beam falls to one quarter of its original value. As a result, increasing SID reduces the amount of radiation reaching the detector unless exposure parameters are adjusted.

SID also affects geometric unsharpness and spatial resolution. A longer source distance makes the X-ray beam more parallel when it reaches the patient, which reduces penumbral blur and improves image sharpness.