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Minimum object distance and close-up rings

Minimum object distance (MOD)

Another important term that is always quoted by lens manufacturers is the minimum object distance (MOD). This is the shortest working distance between the front lens and the subject at which it is still possible to focus using the focus ring on the lens.

Parallel incoming light rays from the object all meet at the focal point of the lens on the sensor side. If the test object approaches the lens, it must be refocussed, as the generated image increasingly moves away from the lens with a shorter distance. When focussing at close range, the lens group is therefore moved away from the sensor.

The MOD is heavily dependent on the lens design and the mechanics of the optics, and is highly manufacturer-specific. For example, classic standard lenses from one manufacturer may have a MOD of 200mm, while other lenses from another manufacturer may have a MOD of 350mm.

In principle, the minimum object distance depends on the focal length and therefore on the depth of field of the lens. Wide-angle lenses in particular have a low MOD. As the depth of field can be increased by stopping down the lens, the MOD can also be reduced slightly. However, this is better achieved by using extension rings.

Close-up rings to undercut the MOD

Inserting close-up rings between the connection thread and the camera increases the image width, i.e. the distance between the sensor and the lens group. This trick can be used to undermine the close-up limit of the lens: By further reducing the working distance, the object field size can be further reduced.

This makes it possible to view small image sections (e.g. 9mm x 6mm) with simple standard lenses and can therefore be a cost-effective alternative to a macro lens. Extension rings can be used universally with a wide range of lenses and are part of the standard equipment of an MV expert.

Use of extension rings

Meaning of Minimum Object Distance (MOD)

By skilfully arranging and combining lenses, lens manufacturers are able to produce affordable lenses with as little aberration as possible at a reasonable cost.

Depending on the focal length, converging and diverging lenses of different refractive indexes are used and combined in different ways. The position of the system aperture and other mechanical edges of the mount also affect the cutting of the optical path at the edge of the image and therefore have a major impact on image quality and distortion.

In addition, lens groups can move differently in relation to each other or to the sensor position during focusing to ensure optimum image quality.

Typically, lenses are designed to maintain image quality up to about 1:20 magnification. Artificially reducing the close-up distance will increase the size of the image, but will also increase distortion, astigmatism and coma.

It is up to the user to decide whether the standard lens can be used as a low-cost macro lens with extension rings and still meet the requirements for error detection and measurement accuracy.

Usage of extension rings

  • The smaller the working distance, the more close-up rings must be added. A wizard for the calculation of close-up rings / lens spacers can be found in the "Service" area.
  • The larger the close-up limit for the optics used, the more close-up rings are required in order to view the same image detail.
  • The more close-up rings are added, the more the luminous power of the optics decreases. A calculation of the effective f number can be found in the "Service" area.

     

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