The MC-21 Mount Converter enables the use of Sigma EF-mount lenses on L-mount mirrorless camera bodies, and maintains full lens performance, including autofocus in AF-S mode and auto-exposure, as well as in-camera correction and image stabilization technologies. An integrated LED can be used to display select Sigma lens compatibility with the adapted camera body, and the adapter also has pre-loaded lens data to support peripheral illumination, chromatic aberration, and distortion corrective functions. In order to maintain a high degree of image quality, the interior of the adapter is flocked to reduce reflections, and it also retains full EXIF data to benefit post-production and file management. Additionally, a removable tripod mount is included for more stable mounting when working with heavier lenses.

This Mount Converter/Lens Adapter is compatible with a select number of Sigma's own EF-mount lenses, and may or may not be suitable for use with other manufacturers' EF-mount lenses. Below is a partial list of confirmed compatible items.

Cameras
- Panasonic S1H
- Panasonic S1R
- Panasonic S1

Lenses
- 12-24mm f/4 DG HSM Art
- 14-24mm f/2.8 DG HSM Art
- 24-35mm f/2 DG HSM Art
- 24-70mm f/2.8 DG OS HSM Art
- 24-105mm f/4 DG OS HSM Art
- 60-600mm f/4.5-6.3 DG OS HSM Sports
- 70-200mm f/2.8 DG OS HSM Sports
- 100-400mm f/5-6.3 DG OS HSM Contemporary
- 120-300mm f/2.8 DG OS HSM Sports
- 150-600mm f/5-6.3 DG OS HSM Sports
- 150-600mm f/5-6.3 DG OS HSM Contemporary
- 14mm f/1.8 DG HSM Art
- 20mm f/1.4 DG HSM Art
- 24mm f/1.4 DG HSM Art
- 28mm f/1.4 DG HSM Art
- 35mm f/1.4 DG HSM Art
- 40mm f/1.4 DG HSM Art
- 50mm f/1.4 DG HSM Art
- 70mm f/2.8 DG Macro Art
- 85mm f/1.4 DG HSM Art
- 105mm f/1.4 DG HSM Art
- 135mm f/1.8 DG HSM Art
- 500mm f/4 DG OS HSM Sports
- 17-70mm f/2.8-4 DC Macro OS HSM Contemporary
- 18-35mm f/1.8 DC HSM Art
- 18-200mm f/3.5-6.3 DC Macro OS HSM Contemporary
- 18-300mm f/3.5-6.3 DC Macro OS HSM Contemporary
- 50-100mm f/1.8 DC HSM Art
- 30mm f/1.4 DC HSM Art