kobalux 3.5/28mm
A compact 28mm lens for Leica M39 screwmount cameras,
the Kobalux is a good companion for my rangefinders.
It is shown here with lens hood attached.
While I've not conducted any scientific testing, anecdotal evidence tells me that this wideangle lens does not suffer significantly from any distortion issues.
More information about this discontinued series of lenses can be found
at Dante
Stella's fine site.
Sample Images
A folder of pictures I've taken with this lens is here.