"enko-Tokina has added the mirrors back into mirrorless with the launch of an ultra-compact Reflex 300mm F6.3 for Micro Four Thirds. This fixed-aperture, manual focus lens revives the catadioptric lens design that was especially popular in the 1970s and '80s for producing small high-magnification telephoto lenses. With a 55mm filter diameter and weight less than 300g, this is possibly the smallest lens of this type that's ever been made for stills cameras. The spec is rounded-off with a minimum focus of 0.8m and 0.5x maximum magnification, making the lens potentially interesting for chasing insects and the like, just as long as you can hold it steadily enough.
The catadioptric design allows long effective focal lengths in small packages but does give characteristic 'doughnut bokeh' patten of sharp-edged rings in defocused areas."