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Lens calculator - Calculating close-up rings / Lens spacer rings

If a very small working distance (= object distance) is selected, small image details can be inspected. However, it is impossible to focus as close as you like, the lens has a close-up limit, also called minimum object distance (MOD). If you purposely want to deceed it, close-up rings must be added.

lens spacer ring = f' * (beta-betaMOD)

calculation of lens spacers/ extension tubes for optical systems

Note: Even when using the drop-down lists own values can be entered. Please use the first entry "user def."!


Sensor size:
Focal length of lens:
Object size/ field of view in mm:
Minimum Object Distance (MOD) of lens in mm:


Calculated working distance:
Needed extension ring:


Attention to small working distances! These equations calculate the object distance (from the object to the principal optic plane in the lens) and not the free working distance from the "front object edge". Please also observe that the effective focal ratio changes when inserting close-up rings!

Further details on MOD and close-up rings can be found in chapter ""Minimum object distance and close-up rings .

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