I dun really know your age but your problem for a man of middle/older age can be due to many problems commonly such as vision degree change, cataract, presbyopia (lau hua) amongst others. Best to see an eye doctor soon especially if your left eye suddenly developed this problem only, and/or you are seeing a lot of flashing lights and floating particles in your eye. He can help you exclude some serious but not so common problems with your eye.