Wide angle lens with blurry corners !!

Yep, as others said the wide-angle converter is to blame. The generic wide-angle and telephoto converters which many sellers include in lens packages are generally very low quality and a waste of money. Wide-angle converters are specially tricky and even those from good brands are rarely satisfactory. The wide-angle converter which Sony makes for the 16mm lens is one of the exceptions and works very well with that lens. Though it is not cheap