I heard that it won't affect the result to smoking before you eye exam. But i think your eyes doctors knows exactly what is is and exactly what happens if you smoking before eye exam. So, you shall ask your eye doctors before you take eye exam. He/she will give you the exactly answer you want to know.
According to some research, smoking weed do harm to you lungs and may cause barin damage. Some people even get cancer from smoking. One time smoking may not affect the result of eye exam, but longtime smkoing may could, so it is better for you to give up smoking weed to keep healthy.
Well, that depends on how long have you been smoking weed. Anyway, that habbit may give you some pleasure and relax your mind, at least for a while. But why don't we think it twice before we develop a habbit, is it worth doing? Actually, if your habbit is new, then the result wouldn't be seriously affected, it's ok. But smoking will cause your body a series of symptoms. Good luck.