Conjugate Points

The object point and image point of a lens system are said to be conjugate points. Since all the light paths from the object to the image are reversible, it follows that if the object were placed where the image is, an image would be formed at the original object position. 透镜系统的物点和像点被称为共轭点。由于从物点到像点的所有光路都是可逆的，因此如果将物点放在像点的位置，就会在原来的物点位置形成像。

The two photographs above show conjugate images formed by a lens of focal length 15 cm. In the top photo, the object distance is 23 and the image distance 43 cm, showing a magnification of -1.9. In the bottom photo, the distances are reversed giving a magnification of -0.54 associated with the shorter image distance.