This 0.8x lens gives a 22.5 mm equivalent field of view.
The corners are a little soft, probably due to too short
focusing distance. The test chart wasn't big enough for
such a wide view of field. As a result the camera-to-subject
distance (about 40 cm) was shorter than the width of the
test chart (60x40 cm).

This kind of distortion can be corrected for example with Panorama Tools.
I'm getting decent results with the following Polynomial Coefficients:

red 0.02 -0.057 -0.0017 1.0388
green 0.02 -0.057 -0.0017 1.038
blue 0.02 -0.057 -0.0017 1.0384