The 0.85x Reducer / Flattener was specifically designed to be used with the Sky-Watcher EvoStar 72ED Refractor Telescope. When you add this reducer to your optical train, it will lower the native f/5.8 focal ratio of the EvoStar 72ED down to f/4.93 while dropping exposure time by 38%! It works well with both APS-C cropped sensors and full frame sensor cameras, but is better suited for APS-C and smaller sensors. The field of view measures 5.8 x 3.87-degrees with a full frame camera and 3.79 x 2.53-degrees with an APS-C crop sensor. This reducer / flattener contributes 55 mm of back focus. Included is a thread-on adapter to mount the reducer / flattener to the EvoStar 72ED focuser.