Hello, Your father needs to first stop smoking. Smoking is a known cause for worsening eyesight. Diabetes is also another known cause for damage to the eyes. He needs to be seen by an eye specialist as there is need to screen for cataracts and diabetic retinopathy which maybe the cause for reduced vision. Contact lens is not the right solution for him as of now because his vision will not be stable unless his sugars are controlled and moreover vision loss depends on the cause. Unless he has suddenly lost complete vision in any eye there is not urgency to visit him now. However do take him to an opthalmologist (eye specialist) and get a complete evaluation soon. Take care.