Converts 28mm lens to 16mm fisheye. Sony 16mm Fisheye Conversion Lens. The lens is lightweight and compact addition to the 28mm lens and features a built-in petal-type lens hood to shield the lens from unwanted light.
If you know what you're getting this is a great lens and a decent value—if you're already got the 28mm. It is decently sharp but there are noticeable aberrations off center. You can get better IQ from a dedicated fisheye lens but there aren't many AF options and most are designed for SLRs so they are even heavy and bigger (and this combo isn't svelte!).
The auto-EXIF data for stabilization and post is useful too (the camera knows when you've got it attached and registers the combo as a 16mm f/3.5 lens).