8mm lenscap is the worst lens ever testet at DXOmark

The idea is you can grab a shot if you don't have time to take it off and put on a lens.
I keep a camera in one pocket, no lens, body cap on, and two lenses in two other pockets.  I've lost once-in-a-lifetime shots fiddling about putting on a lens.  Now, I can have the camera ready to grab a (slightly fuzzy) shot in an instant.
Brilliant idea - I shall buy one for each of my cameras.  Next time a mate goes to the US that is, EU price a joke. 
Of course, viewed as a lens it is terrible.  As you'd expect.

--
www.flickr.com/photos/acam http://thegentlemansnapper.blogspot.com