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