= Minimum object distance
Closest distance for which the lens works optimal and especially can be focused.

This does not mean that a lens at shorter working distances no longer “works”. … just: please do not expect the “perfect” picture quality.

Focus on shorter working distances can be achieved by increasing the distance between sensor and lens (= screw type design of the lens and possible use of distance rings)

When working at shorter distances than MOD generally expect:

• The light sensitivity changes. 
 I.e. Images are generally darker because less light reaches the sensor.
• Depending on the very lens design, it may happen that mechanical vignetting occurs, because  unintentionally 
 other mechanical components in the objective take on the role of the diaphragm.
• Field curvature may increase. You can tell from focusing that is only locally, say, focus only on the edge, or just in the middle or only on a ring around the center.