# Tube lens
## Details
*   **Focal length:** 50mm
*   **Outer diameter:** 12.7mm
*   **Material:** glass
*   **Lens type:** cemented achromatic doublet
*   **Supplier:** ThorLabs
*   **Supplier part number:** AC127-050-A

The microscope is designed to use finite-conjugate microscope objectives.  However, it is necessary to shorten the distance between the objective and the sensor by inserting a lens in the intervening space.  This is **not** placed one focal length from the camera (as it would be in an infinity-corrected design) and so isn't really a tube lens - think of it as a "tube length correction lens".  There is a version of the optics module that does this, but it is rarely used.

These lenses can be conveniently sourced form ThorLabs, or less conveniently from AliExpress at lower cost.

The tube lens is only required if you are using an RMS microscope objective, i.e. in the high resolution version of the microscope.
