In lens selection, the most important factor in determining the relative magnification of an object...

Focal length is the primary factor in determining the magnification and field of view in a camera lens. A longer focal length results in a narrower field of view and greater magnification of distant objects. Conversely, a shorter focal length gives a wider field of view with less magnification.

A (Image device) and B/C (Lens or image format) influence compatibility and image quality, but do not directly affect magnification.