对于光学图像，透镜方程定义了物距 g、像距 b 和焦距 f 之间的关系：
1/b + 1/g = 1/f
对于薄透镜，通过几何推导透镜方程，并将物距 G 和像距 B 包括在内，可以定义以下放大率：
放大率 A = B/G = b/g
下面的图可以更清楚地说明这一点。
对焦意味着改变镜头与图像传感器之间的距离，而图像传感器应位于图像平面内。然而，镜头只能从无限远（最大物距）对焦到最小物距（MOD）。
For an optical image, the lens equation defines the relationship between object distance g, image distance b and focal length f. It states:
1/b + 1/g = 1/f
For thin lenses, the following magnification can be defined by geometrically deriving the lens equation and including the object size G and the image size B:
Magnification A = B/G = b/g
The following figures should make this clearer.
Focusing means changing the distance between the lens and image sensor, which should be located in the image plane. However, a lens only allows for focusing from infinity (maximum object distance) as far as a minimum object distance (MOD).